"Grass is greener" syndrome. Believe it or not, this isn't uncommon. You're happy in a relationship, but wonder if there is more out there. If you're young, you might want to act on those thoughts and go play the field a little bit more. The question is, do you think you could be happier with someone else? If yes, then go for it, give it a shot. Don't settle and sell yourself short.
I left a long-term relationship when I was 25, we had been together 3 years, the prospect of marriage came up, I was happy, but something was missing. I decided to walk away from it, I was young and doubtful. I was single for a year, dating a bit on and off, had a couple of short relationships, etc. almost 2 years after the breakup, I found someone new, someone who is everything I ever imagined - I took the chance, and it paid off (to date, it has only been 6 months, but thebest 6 months of dating I've ever had).
"All is fair in love and war." - Francis Edward Smedley