It's an inherited crap from generations ago, long before one's greatgrandparents were born.
Some are imposed and then passed down, some are naturally had done and turned into passed down expectations.
All in the name of survival; be it the self or the genetics, the name or the society sustaining itself.
And thus; whatever social expectations there are today, are likely born from survivorship in the past.
Like being rich and getting laid guarantees survival and passing of genes.
The social optional bits? Optimization of said survival through social fit, in which would either garner more empathy from others or more notoriety.
All of what I said is very much likely implied and not thought about much.
In which outcomes condensed into everyday thoughts of what society thought something is right or wrong when it came to milestones.
Everyday layperson language would say "you're a loser" or "not good enough" for not fitting whatever social expectation.
Underneath that would be an implication would be "your chances of surviving and passing down your genes is lower" and "best focus our resources to those with better potential who do".
The shaming and guilting is supposedly the mechanisms to drive someone to "do better"; with the counterparts of approval in conforming to said expectations; it's just another crap inherited and created by generations of socialization, really, in the name of keeping something alive -- be it a made up ideal or something evolutionary grounded.
Really, I do think, at the start it's something necessary or adds chances to survive, it just became something akin to what is "good" or "right" and therefore "moral" as it "normalizes".
Be it the society created turned more symbiotic, exclusive, capitalistic, or whatever.
Even if it's no longer really something that necessitates and/or adds chances of survival.