I feel like the biggest issue when it comes to relationships especially as you get older, is time.
You have to invest your time into another person in order to maintain what you guys have.
Whether it be being in school full time, working, traveling, whatever it is that you're doing for yourself, it will always be a problem.
Friends/family/partners are so quick to root you on for doing good and focusing on yourself, however, are quick to complain about the lack of time you have for them.
This is mostly an issue with growing older, and is why I have a very small circle.
They get upset because you're too busy to talk, hang out, go on adventures, etc like you guys used to, and become more distant.
Time will probably be the biggest factor as to why you don't have many left around, honestly.