do people really get upset about this?

Yes. I’ll dive in, assuming that the greentext is real; the scenario is plausible enough.

Romantic rejection is painful, as it’s based on an instinct to achieve a strong mutual bond with someone else. This may or may not be conflated with a drive to reproduce, depending on the level of sexual attraction involved. The sensation of loss here, can manifest in actual physical pain in one’s head and/or viscera, and is proportional to the level of “drive”. This also gets coupled with a sensation of loss as the reward for achieving that mutuality is a moment that is usually followed by intense pleasure (even without sex); suddenly realizing that reward isn’t coming, hurts.

The second part, where the group continues without Anon, is similar but a different phenomenon. It’s rejection from the entire social group. Our instincts to be social creatures causes us to feel this as a loss (painful), because we’re safer and stronger in groups. Instinctively, the sensation will subside once Anon figures out how be confident with being alone, or (more likely) finds a more compatible social group.

Attempting to introspect the above sensations without support can also go to bad places. Anon mentions his self-esteem - they are blaming themself since that’s a position of “control”, but ignoring the reality that this was all impossible to predict or avoid. In reality, the other partygoers are a bunch of insensitive assholes and carry 100% of the blame here. This person really needs to be around people with more empathy.

Combined, Anon is in a world of physical and psychological pain. They were denied a potential romantic and/or sexual reward, and were rejected by the entire social group. Both forms of rejection provoke instinct and our reward centers in ways that just make a person miserable.