That's because we have been studying for what? 16 or 17 years? That is a lot of time doing the same thing. I know subjects change, schools change, systems change. But truth is, all you do is study. And probably when you get to 22, your mind is over that, you start being too mature for that. Only caring about studying and exams and similar stuff is not enough anymore. But it's bound to change for both of us rather sooner than later. That is the good (?) news.
Also that thing, you know, something you haven't found that is supposed to make you feel fulfilled and complete? Something we think that can be a job, a life partner, a kid, moving to another city/country, starting this or that project? That does not exist.
Nobody ever feels 100% fulfilled, and if they do, hell they are the luckiest people on earth. When you find that thing, and you may know what it is so you know when you find it, you will find a way to feel incomplete again, to start craving for something else. It's just life.
I mean look at me. For 3 weeks all I wanted was time to relax and do nothing. 2 days into my holidays and I am freaking out because have nothing to do!
Some rare cases apart, the human race is naturally dissatisfied.