I believe I have a legitimate reason for breaking up with my girlfriend, but friends think otherwise. What say you?

After 6 months of dating my current girlfriend, I have slowly come to the realization that I'm not perhaps as great a fit for her as I thought. While we have similar interests, sense of humor, and general attitude, I feel as though my level of simple, passionate desire for her isn't as high as it should be. My friends say that I don't appreciate what I have, but I think I'm simply not as satisfied with a SO as I should be. Your thoughts?