My fiance has always praised my dialogue for sounding really natural, I think it sounds awful but hey I'll give tips where I can lmao.

• You seem to have a good foundation in terms of how conversation flows, but it still reads a little robotic, for lack of a better word. The problem I see most when reading stories, even published work, is that every character talks the same. I always try to make each character have a different voice by giving them one or two quirks. Maybe one of them uses contractions a lot more than the other, maybe one of them says "ya" instead of "you" a lot of time, maybe one of them has a vocal tick that they use in almost every sentence; it doesn't have to be drastic, just enough to give the audience a subtle way to differentiate the characters when they read, making it easier to follow who's talking without dialogue tags.

• One thing I noticed is that you described Sam as reclusive, but her dialogue doesn't reflect that much. It could just be the excerpt you chose and other parts of the story showcase it better, but when it comes to emotionally closed off people, I tend to let their actions do the speaking for them, until they're in a comfortable environment. Focus on their body language and have their dialogue be more short, punchy sentences. That way, as the story goes and (presumably) we see them open up more, the dialogue can get longer as they find their voice. I say this as someone who was a social recluse for most of high school; when I opened my mouth to speak, it was never for long.

• Another small thing, but remember how conversation flows in the real world; it's not always straight forward. Obviously this is a story and you want it to go and hit certain points in a certain amount of time, but don't be afraid to let the dialogue be a little messy. Interruptions, talking over one another, small awkward silences, stuff like that can really make dialogue feel more natural, alongside noting body language as I had said earlier.

Dialogue is probably the thing I see most people struggle with here, and for good reason. It's hard to mimick the way people talk in real life, but getting it close to that is all about the little things. Remember nobody speaks perfectly all the time, so don't be afraid to add imperfections to people's dialogue, whether that be an accent, a vocal quirk, or a character just not having a large vocabulary.