DESPITE THIS THE WARNINGS YOU DO NOT HEED
Speakin' of lives, shouldn't you be livin' one during noon EST?
I don't know. There's no reason for it to have broken. I noticed that it was messed up while playing SMT IV where the the R or L button allow you to see info on demon moves. L worked fine, but not the R button. I bought the 3DS around September of last year, so it's unexpected that I'd have this much trouble so early on.
If ya ain't dropped it or nothin', perhaps the factory just dealt you a poor hand... or maybe the button is jammed. Prolonged exposure to dust/smoke/filth in general can do that without necessarily makin' the button stick or feel like it's not workin'.