Why am I So Sad?

For some reason I have this sense of sadness about me and I'm not entirely sure why.  Things at home are fine - the boys are going to their preschool classes and are doing well.  And, the hubby gets a little time to himself when they're at school.  Everything seems to be going well.  So, what's the problem?

I'm not entirely sure.  All I know is that I have this sense that something is missing in my life.  I wish I could be happy with the way things are, but I've always been the kind of person to wonder "what's next" or "is this all there is".  It is a little "Wizard of Oz"ish I suppose because I'm always wanting more than what I have.  I just need to learn to be happy with what's in my own backyard.