I have a decreased sex drive, what could be some of the causes?

I am 24 years old and over the past year my sex drive has dropped dramatically.  I have been dating the same girl for two years, but as of late I have had no real desire to have sex and at times it seems like a chore.  I enjoy the actual act of sex, but the majority of the time it is not something I go out of my way to seek.  Prior to this year, I had what I feel was a very active sex drive, but this last year has decreased my drive drastically.

Thank you in advance,