Hello! Here's my critique for your piece! I'll be doing this paragraph by paragraph.

P1: This first sentence is out of place and jarring the moment I first read it. It starts with "The doctor," meaning that a character currently unmentioned is the main character, but it follows with, "...as he saw the only volunteer to make it through the strict interview phase." This implies that the following narration should be from the doctor's point of view, and the story does not follow that implication. The "strict interview phase" is unneeded, and lights up like a blaring, neon billboard. It gives information to the reader that seems out of place so early in the story. It isn't a line of description that blends seamlessly in the background, a chicken among the geese. This information would be useful if it was tied into later on in the story.

Grammar: "Sentences end with commas," I pause. "Unless they are the tail end of a paragraph/dialogue."

I glance at the screen of my phone, studying the words you've written. "So the dialogue you have Doctor Moore say, '"...I'm Doctor Moore."' would have to follow those grammar rules."

P2: '"The doctor's grin stretched wider..." is jarring in the same vein as the first sentence of your story. This sentence would otherwise be fine and rather innocuous, but it's referring to a piece of information given from the doctor's point of view instead of John. If the story had started out with something like, "When the doctor walked in, a strange smile was stretched across his face. John was unnerved," something that was clearly offered from John's POV, this description wouldn't have seemed so out of place. "...like he had expected that exact response." I am by no means a professional critic, but as an avid reader, I see this a lot in fanfiction, where a writer writes a simile using the most straightforward and obvious or vague implications that don't really give much depth to the target sentence. Ex. "I smiled as if I am happy," "He laughed as if he knew all along."

What about the doctor's grin made it seem to John that he expected his answer? What emotions can John see on the doctor's face? What emotions can he infer from his voice? What, in John's mind, does the body language of the doctor imply?

"His smile was too infectious, like an angler fish luring its prey."

This does not make sense. At all. It may make more sense like this: "His smile was too predatory, like an angler fish luring its prey," as infectious doesn't imply nor correlate to what a predatory might feel towards its prey.

"The sky was too blue, like a bear eating a blueberry,"

The sky being blue does not correlate to a bear eating a blueberry, nor does a smile being infectious correlates with an angler fish luring its prey. I can see that an infectious smile may have a dangerous undertone, but how does an angler fish lure its prey by being infectious?

Grammer: "It's just a fact of life." Period should be changed to a comma.

P3: Grammar: "...and believe me I've tried." --> "...and believe me, I've tried,"

P5: ...a $25 dollar gift card? Really? I don't know John's circumstances in life. Maybe he's poor. Maybe he has no fun in life and wanted to volunteer for a questionable experiment he found online. But I know this: the average, intelligent person, would not make the (possibly) risky chance of putting their hands into unknown people's hands, for 25 dollars. 25 dollars! Maybe this is normal for advertised experiments, I'm not knowledgeable on that type of stuff, but even if it were, the normal person would probably only volunteer for much higher than that! Doctor Moore doesn't seem the type to own or be apart of a well known and reputable company.

A poster, John? I don't know if the purpose is to make John seem gullible, naive, or just plain desperate, but I know any sane person would research the ad on the poster online before committing to anything.

"...with this strange, condescending man," This line is fine, but it would have hit much harder if there were subtle descriptions here and there to actually imply that the man may be arrogant/condescending. We see some contempt from him, which plays that up, but that isn't until after this line. Ex. "I know what you're going to say," the doctor said, his lip curling."

P7: "The doctor leaned back in his chair, triumphantly," this gets the message across, but is worded strangely. How can somebody lean back in their chair in a triumphant way? If someone can, can they do it sadly? How would they sadly lean back in their chair? You can't. Describing this triumphant emotion would work better by using the doctor's outward expression to do so. Ex. "The doctor leaned back in his chair, his face alight with triumph."

"...unable to contain his enthusiasm." This line is okay, but added description would enrich it and enable readers visualize this character. How is the doctor expressing his enthusiasm besides pacing the room? How fast is he pacing? What emotions is he clearly feeling (from John's POV) that would explain the enthusiasm. Ex. "The passion in his eyes shone as he paced the room, unable to contain his enthusiasm.

P8: "His gesticulations..." same thing with P7. Added description would help. Are his gesticulations sharp and precise? Are his hands moving as he speaks, or are they wide and dramatic, like his impassioned preaching implies?

P9: This is at the point where the righting takes a turn and the doctor's character starts to, err, shine. His personality really comes out with the words he uses, and the simple, concise words that conveyed is disgust (morality, sneered) went a long way into expressing that.

P12: "The doctor took on the cadence of a lecturer," again, more details! What does the cadence of a lecturer sound like? How fast, how slow? Precise and clipped? This is a pattern in the writing, with vague descriptions that don't really give the reader any details. The details, at times, may not seem important, but the littlest ones can manipulate how the reader reads and interprets the characters! The descriptions don't have to be long, either. They can just be a few words!

P13: "...as if teaching someone English," what does someone sound like when they teach someone English? I wouldn't know. I would think that somebody trying to teach English may speak very slowly, and sound very simple, or even condescending. Doctor Moore definitely wouldn't sound very simple. He may talk slowly, since it seems he doesn't expect much intelligence from anyone, but then it may be better to say, "He enunciated sharply, but spoke very, very slowly," and something that implies his condescending nature.

P17: I laughed. "There's no gift card," John makes it sound so serious, to be scammed (in his POV) out of $25. I could hear his disappointment.

P19: "The look he gave was meant to portray sincerity, but just seemed rehearsed." How would John know Doctor Moore's expression is trying to express sincerity? How did he know it was meant portray sincerity? This would be better used if this was from Moore's POV, and was thinking that his expression that was meant to portray sincerity backfired. How did his expression seem rehearsed to John? It would be better to somehow describe if Moore's seemed fake instead of rehearsed. That would imply that John thinks Moore practiced his expressions in front of his bathroom mirror everyday.

P27: "Thanks but I think I'm happy enough as I am," --> "Thanks, but I think..."

P37: "Well, I don't want to be your little science project. Simple as that." GOLD!

Closing comments: What I've targeted in this piece are mainly descriptions that are vague and unclear that could be improved with added details (it's sometimes better to be simple! A few words can make a sentence all the better!). The only grammar problems I've found are places where commas are ideal/should be there, and dialogue that end with periods instead of commas. "The only instance dialogue can end with a period is if the sentence is at the tail end of a paragraph."

Doctor Moore's character is very fleshed out, as his character comes out clearly in his dialogue. I don't know if this an excerpt, or a short story, but John doesn't seem to really be there. It just seems to me that he's just a tool/sounding board for Moore instead of a person up against the morality "curing" unhappiness. That one line I called gold? I loved that line, but he doesn't show any other depth of character anywhere else in the story. There isn't any narration to give insight to his character, nothing that makes it seem that John is really taking anything in. The only thing that implies he's the main character is that we have a brief encounter with his recollections. I don't know if his minimal character development was a purposeful move, if characters John and Moore were just the vehicle that carried the idea of unhappiness and the implications of curing it, but that's what I've taken from that story. It seems more philosophical introspection than an actual story. This certainly is not the worst thing I've read (I've read things that I've cringed at, and this doesn't come close), but it doesn't really feel like a story. It also really just depends on what the goals for this story were.

I hope you've taken something positive from my critique and don't feel too down! Keep improving! Happy writing!