Philosophers have pondered that question for centuries. I'm afraid the answer is disappointingly simple: Mating.
That's it.
Christians seem to think that life is a test, and that the goal is to get into Heaven. But that's like saying your job is to get a promotion. No, your job is to work. And then, if you worked hard, then you get promoted. Heaven is supposed to be a reward or promotion, for a job well done. And what's our job? "Be fruitful and multiply." We are here to mate and procreate. That's it. That's all there's to it.