Well, let's see......
I played BECMI from around 1983 until 1990, from the time I was 8 until I was a sophomore in high school, as well as some Top Secret / Top Secret S.I. Also played quite a bit of Battletech, and I LOVED all the Mechwarrior video games.
I then literally didn't play a single organized tabletop RPG for 13 years, until 2003, just after 3.5 came out, when I was nearly 28 years old.
Why did I stop playing?
Lots of reasons, mostly that I just liked to do tons of other things. I was never the type of kid/teen to just throw my whole life into D&D (and let's be frank--by about 14/15, it's not exactly the "thing to do" to impress the ladies). I played basketball, volleyball, golf, and tennis, got into acting in high school (and did it in college and semi-professionally), learned to play the piano, and generally just enjoyed high school life.
I should also mention, though, that I kept all of my original gaming material--Battletech stuff, my original Basic/Expert soft covers and my Rules Cyclopedia, all my dice, my character sheets.....
I also played D&D video games regularly, from the SSI "Gold Box," to the Heroes of the Lance early PC title, to Baldur's Gate I and II.
I came back to the hobby because I had a friend I worked with who had an adult gaming group. As soon as I got an invite, I fell right back into it, bought 10 or so of the 3.5 hard cover books, FR and Eberron campaign settings, minis, maps, etc.
Now it's just part of my daily lived experience. My wife has played with several of my groups (though she really only does it because she knows I enjoy it), and I'll always have a bookshelf area lined with my RPG stuff.