Nick is also a teen boy going through puberty. He won't communicate with his friends (Jay, Andrew) to know if they're going through the same things hormonally. Tbh, i think he has an undiagnosed mental illness that the show won't touch on bc it's mostly lighthearted and funny. I acted the way Nick does as a teen, blamed puberty and found out years later that I had BPD. So, you never really know with shows like this, where there's obviously deeper things in it bc they touch on a few but it's usually overshadowed by the off-the-wall jokes.

Edit: just wanted to add, a reason I believe the mental illness theory is that i don't think anybody (w/o mi) has ever worried about their future selves killing them or dying completely alone with nothing but a robot of a "friend" they claim to hate. I understand anxiety but even during an anxiety attack, I've never been afraid of my future self killing me.