Perhaps, I wondered just now, I have trouble writing sad scenes because I'm never sad? Hmm.
Humor -- often snarky, sarcastic humor -- comes easy to me, for the simple reason that that's the sort of person I am. But I rarely get sad. (Not to mean I don't cry; show my the right movie, for example, and I'm a river)
Possibly I'm enjoying too wonderful a life to be able to create the sort of scenes that would complete me as a writer. I may just have to live with that.