> I don't understand why people dislike working in a startup environment

Different people appreciate different things. It's totally fine for you to value "regular parties, play table tennis daily, beers on Friday" but you should be able to understand other people value other things.

> Admittedly, this is only my second job, and I am only 20, but I can't imagine ever wanting to leave.

You may feel differently in ten or twenty years. There were probably a lot of things you enjoyed when you were 10 that you don't enjoy now (or don't enjoy as much).