It actually is not just a part, but the whole concept of growing up is sad.
We get a job which is actually what we had been wishing for, to pay own bills, do own taxes, to earn. But what we never considered about getting a job is having to get up by the time we used to go to bed. Having everything scheduled. Working, studying, sleeping, eating, making healthier food choices & actually having breakfast on time. Being responsible or at least claim of being. Showing up at home on festivals that too just important ones. Taking our own care when sick beacause we, in most of the cases move out of our parents place which sounds cool at first but sucks in the long run. We no more have to get permissions for hanging out, but we no more want to. We love being the couch potato & cancel all those plans which include getting dressed up & moving out of our room. We start withdrawing from fun activities that we once loved & actually block the ways that possess some tendency of making us social or happy. By being social all we understand is interacting with the people at the place of work & these people even get into our speed dials,where once used to be friends. Those friends which we know nothing about today, cause we have ended up abandoning them, not intentionally but eventually. We start loving social networking sites as they understand our intrest of replacing actual personal contact & there we find news about our friends getting married, moving to a new place, getting awesome packages, qualifying exams. To keep it simple we find ever body else's life interesting. In this phase we hear the words marraige & divorce instead of hookups & breakups. We congratulate our friends of getting pregnant instead of freaking out, thats when we realise everyone around is growing up.
Others signs include (of growing up) actually worrying about future instead of worrying about the weight or looks, falling in love with sleep & food, using computer & phone for Real Work, watching news bulletins & weather reports. Being mature & even if you are not pretending to be one. :)