We should start off by saying that partying is optional. It does not necessarily make you cooler, more popular, etc. (not to say it can't, it certainly can); it is just another one of the jungle of events you can experience in the wonderful world of college.
If you really want to do it, you can do it later.
That's the anti-FOMO chant, and it is hard to understand upfront. There are also some practical factors to it, e.g. usually it assumes you have the time, resources and health to party or do whatever you wish later on in life. The issue later in life is that you don't have the environment or friends to do the activities that you could have in college.
But for the most part, not going out with your friends one night or missing some concert during college is not going to cause huge regrets. It's just in the moment, while heading home to your dorm in the dark does feel bad, since you might be conjuring hypotheticals about how fun it would be to go out and all. Honestly, if that's what you're thinking, just go out; the thought of FOMO is probably going to cause you to be unproductive anyway. But work to have more independence over time.
Everyone matures at different ages, so if you don't understand why people around you are doing certain things, that's ok. The flip side is, you shouldn't actively avoid everything people are doing. It's just, if you don't totally get it all the time, that's ok. But you should strive to get it eventually.
There is a blend of being too uptight, where you reject all hang out requests and things to do, and being too chill, where you yes-man and do everything others say. As with most things in life, there is no correct answer for where you should stand, and people can find comfort and success along that spectrum of chill-ness.
You will inevitably do things that hurt others or yourself that you will regret. But, just think of it as "the process" for growing up. It's all part of the happy path to adulthood.