I do think some (maybe most) of it is luck/brain chemistry, I feel happy a lot as I get older. Part is just that deep appreciation I feel when I wake up and realize that instead of school I will go to a job that pays me. Having kids was stressful but absolutely did increase my enjoyment in life, my desire to live, if that makes sense. More good than bad by a large margin.

Good news is if you are 40-50 you are getting to that age too - news articles say it's like we sit back and enjoy the fruits of our labor but I think bullshit because I can't slow down yet and still feel it, it has to be changing brain chemistry and perspective - happiness comes easier now and also fewer things irritate me, youth is an irritable time.

And I guess finally, I really do think luck plays a big part - not in outward circumstances (though obviously luck is very important there too, circumstances don't guarantee happiness) more in the ability to feel certain things. So my actual advice is to adjust perspective if you can, be grateful for the things you can, get physically active to the extent you can and take time to do pleasurable things because even if you are not wired to feel that rush of "happy" you may still be able to feel content and thankful and good.