That isn't the reason, though. On my part, at least, it's that Joss CONTINUES to do it. Over and over again, at least once per season. Other writers don't go there with the relationships in the first place, but he goes there and CONTINUOUSLY breaks them apart. By death, by someone leaving town, etc., he continuously writes his characters in some kind of relationship heartbreak. On EVERY show. Well, technically, it didn't happen with Firefly until it became a movie, but still.
It is old hat now. At first, it was shock value, it was something new and shiny to look at and go "Oooo, hey! That's pretty awesome! That kinda stuff isn't really done." But now, he does it on every show, every season, every time. I adore Joss, and that kind of stuff is fine...Once in a while. I can't think of a season of a show that he's done that he hasn't completely killed a relationship. He needs to figure out some new material, pronto, 'cuz now it isn't shocking, it isn't compelling, hell it isn't even heartbreaking; You see it coming a mile away, and rather than leave us in shock, awe, and utterly heartbroken, it fills us with something else...Something that Joss usually isn't capable of filling us with...Boredom.