Seriously though. Relationships seem to be a major focus in horror. For instance, throughout most of the early 80's, you could tell who was going to be the next victim based on their inability to hide their feelings for each other. (Although, I suppose you could also blame their stupidity for rushing off into the woods alone to get it on.). A good example of the different kinds of problems relationships can cause is the Friday the 13th series. As I said, in every movie, there's at least one couple that dies together while they're in the middle of coitus. Killing two people with one spear, as the saying goes. Though, you can also say that the whole series starts over some guy's attempts to get into some hottie's pants. If that one lifeguard had been paying attention, Jason might never have drowned and gone on to become one of the most bitter zombies ever to walk the Earth.
It's not just being in a relationship that can get you killed though. You can also end up on the bad end of a large, heavy object just by being desirable. Who can forget the rabbit scene in Fatal Attraction?
Dramatization. No rabbits were harmed in the making of this blog post.
Of course, you don't even need to be the specific object of desire to get hurt. In Stephen King's Misery, writer Paul Sheldon isn't even the object of Annie Wilkes' obsession. He's just in between her and her love, and for that, he gets an ax to the ankles. (It was a hammer in the movies, but in the book, he actually loses his feet entirely. Sorry if that's a spoiler to anyone.)
Of course, while not so prevalent in Horror, you do also have those stories where love conquers all. Where the bond between hero and heroine is the only thing that can defeat the darkness and spare their lives. More often than not though, in Horror, the hero has to make a choice and decides to sacrifice himself in order to ensure the safety of his love. Of course, it doesn't always work, and some lucky(?) guys might only get partially mutilated before the girl rides in with the cavalry to save his ass.
Though, let's be honest here. Few things really suck more than being by yourself in the middle of a huge number of happy couples. Just ask Carrie.
But yeah, totally not bitter.