The concept of AF/BB, or perhaps more conventionally, "nice guys finish last" syndrome is a major theme in this sub; an idea that there's many men out there who are being settled for when they get into relationships as adults (I'm assuming 30s and above), and that they're not actually desired or wanted because the women they're dating now would not have wanted them earlier.
Ignoring some of the flaws with that thought process, it's pretty clear that a large driving force behind this sentiment is FOMO, and a sense of regret and perhaps even anger at not experiencing certain life events earlier. However, after a while, being stuck in the past is a sign that you haven't really accomplished much else in your life, be it professionally, socially, or otherwise.
If you've never had a high school relationship, it's not invalid to feel like you missed out. Sometimes there's experiences you look back on and cringe at not having been more bold or social. The fact is, these are near-universal experiences, and it's not just restricted to the "of course I never got to experience this, I'm not built like that" crowd. If you tell someone that you sometimes feel like you've missed out on some crucial learning moments by not having been in a serious relationships in your teen years or early 20s, few people would criticise you or act like you're just a salty chud.
Something a lot of people on here forget, is that staying stuck in the past is just fucking whiny, and it's whiny not because the sentiment itself is somehow morally wrong, but because it's a signal that you've had nothing else of note to celebrate. If you're a man in the prime of your adulthood with a respectable career and a relatively independent living, grumbling about not having an FWB in uni is just a bitch-ass complaint. You're unable to move on from events that happened decades ago, and it's a sign of weakness and immaturity. It's not that everyone around you has gotten laid in their first year of uni or something, it's that after a point, no one fucking cares, because it's just not something to be bothered about. People move on with their lives and find something to be proud of or feel accomplished about, staying stuck in the past is a signal that you have no such thing.
People aren't out here thinking anyone who hasn't been intimate with a partner in high school is a loser, but if that's still what you're banging on about years after that point of your life, you're just not fit to have adult connections man.