It's the same thing that happened with Buffy and Spike: Oooh, a dark, interesting relationship where the characters totally spark off of each other. Quick, let's make it as dreary and miserable as possible!

I don't know if it's a sexist thing (we can't have our precious goody-goody heroine actually enjoying herself with a "bad boy") or a moralistic thing (if this "bad boy" is going to be involved with our lead, we have to sand off all his rough edges and turn him into a wuss) or what....

But, hey, they're the ones losing viewers, so their loss.