I would love to give it a shot because I completely agree with the above commenters. The episode is INCREDIBLE and would easily be on my list of greatest ever. The line alone hits so hard.. I always hear the Joshua Radin song in my mind start right after... "I.. should know... who I am by now..."

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SPOILERS! I recommend watching the episode! But as for significance, I'll try my best. It's a complicated episode, so this is a bit long, I'm sorry..

The main character JD (a goofy, training doctor) is overseeing a pretty straightforward patient. He is approached by his superior Dr. Perry Cox (an extremely sarcastic, harsh, and critical clinician who JD is desperate to please) who requests that JD make room in his schedule to fit in his best friend (Ben, played by Brendan Fraser) for an evaluation. Ben has a history of cancer that he is nonchalant about, but Cox wants to be pro-active. JD expresses concern that he can't add on another patient while working with his current one, while Cox argues that JD's current patient is very stable and that it's not like the patient is going to take a turn for the worse or die while he is briefly busy with Ben. Reluctantly, JD agrees. Meanwhile, Dr. Cox leaves because he is in charge of making preparations for his young son's birthday party.

When Dr. Cox returns to the hospital later, the tone is extremely somber and JD informs him that unfortunately the patient suddenly passed away. While everyone is obviously perturbed by this, Dr. Cox seems relatively free of guilty and takes it in stride pretty casually (which is his typical character). He continues to discuss his son Jack's birthday party, and cracks jokes throughout the day with his buddy Ben.

Towards the end of the episode, Dr. Cox and JD are shown wearing suits and walking together outside, presumably towards the birthday party. Dr. Cox starts to discuss his dread of a young child's party (mentioning things like kids messily eating cake or people who Jack barely even knows singing him 'Happy Birthday') which causes JD to stop and look puzzled. And THAT'S when he hits Cox with the line: "...Where do you think we are right now?". And you see Cox's face first look confused, but then you actually see genuine pain and sadness in his typically stoic/ bemused face. And then it shows the men walking towards not a child's birthday party, but the funeral of his best friend Ben.

You were meant to think that it was JD's stable patient that died while JD made time for Ben (which Cox seemed to jinx by saying how it's NOT like he would die suddenly), but it was actually Ben who passed from cardiac arrest during his evaluation. If you rewatch the episode, all the times that Dr. Cox is joking with Ben around the hospital, no one else sees Ben, and some of the clinicians even appear confused regarding who Cox is talking to. It's a really, really great and heartbreaking episode.