I don’t think there’s a purpose, really; to put purpose or duty to a sense of joy is to try to quantify it, to package it - and since everyone is different, and perceives things differently, then that means joy takes just as many different forms and can’t be packaged so neatly.

I think life in every sense is more chaotic than that, and that randomization of cells or events or emotions can’t and shouldn’t be whittled down into some universal experience or explanation. Making something your “purpose” immediately brings with it a certain expectation - it almost makes it sound as if you’re trying to be happy out of duty, which seems…weird.

I don’t think you’re failing at life if you’re unhappy. Everyone has low or weak points, and that inevitably changes in some form of another. That’s one of the universals, a product of events and living situations and hormonal/genetic makeup. Sometimes people can have all their needs cater to and still your brain will fuck you over.

So I’d ask yourself why you’re looking for a purpose in the first place. It makes sense if there’s some goal you want to attain; and if that goal is happiness, it’d be best to think about some actions you could do to obtain it. If you’re not looking for that goal and just asking why people seem to pursue happiness, then the best answer I can give you is: why not? Suffering feels bad and I don’t wanna feel bad.