The U-Shaped Curve of Happiness

If you're someone who's a decade or two out from your high school graduation, do you ever find yourself thinking that you're just not as happy as you were back then? Of course all the positive-thinking self-talk then kicks in and you think, "Well, maybe I actually wasn't that happy before. I do like my life better now. I like the independence I have. Yeah, yeah, I really like being an adult." Yet, no matter the glass-half-full glow you try to put on things, you can't shake the feeling that your happiness has declined over the years, that at 30, you weren't as happy as you were at 20, and that at 40, you weren't as happy as you were when you were 30.