The two biggest lessons you learn as an adult:

1.  Keeping your mouth shut is often much smarter than saying something.

2.  It's pointless to be jealous of anyone else, because almost everyone has problems you don't realize.

You may be reading both of those and saying, "Duuuuuuuuuh," but they're not all that obvious when you're younger.  There are still times when I consciously employ #1 because my stupid comment is really unnecessary, even if I might think I'm funny.

And #2 really applies to almost everyone.  There is pretty much no one who has a perfect life.  But when you're a kid, you sure think so.  Your friend's parents seem way cooler than yours, and your other friend has a nice three-story house, and your other friend is popular.  Then you find out years later that those friends' parents were into "wife swapping."  (You only find this out because your dad bumps into one of them at "Parents without Partners.")

Even knowing that it's silly to be jealous, I used to get jealous a lot -- mostly in my twenties.  Your twenties can really be about that.  Some people spend age 21-29 stumbling around, and other seem to fall quickly into a permanent life plan.  You measure your life against those latter people.  You see them in the Times wedding section:  Classmate, 26, a newly minted lawyer (law school only takes four years, you remind yourself), married to someone he/she met two years ago in law school.  They've already trumped you in occupation AND in relationships.  You don't mind too much, because you know 26 is young, but you don't want EVERYONE to get married and figure out their whole career before you do.  It's not always about jealousy;sometimes it's about just worrying that you're not doing something right.  

I had to stop reading the Sunday Times in my late twenties because I was too jealous of too many parts of it.  It started with the weddings page, then went to the book reviews.  And then, jealous of other classmates who were writers and had essays or stories in the Magazine, the Travel section, the New Jersey section...

But I digress.

Of course, ending up in the right place in my life finally stopped me from being jealous.  But I learned early enough that you can't envy someone else's perfect life, because it is rarely perfect.