Its been covered fairly well, but there are multiple reasons why most of my characters end up good (especially if it is some generic binary choice early in the game). My main reason is that I don't like being a dick. There may be times where an in game character has screwed me and I want revenge or something of that nature that pulls me towards a darker path, but generally speaking the evil role is just relegated to dickishness. Secondary to that is the lack of forethought put into evil characters in general (unless it plays into the story), that stops me from picking evil when it is a binary choice, as I know that most evil is done poorly, whereas some good is done poorly.
Really it comes down to how good the game is, and how well it was written. If the game was fun enough, I'll do another play through and focus down evil choices, if the story was good enough, I might even make evil my first playthrough. It all depends on the game in question though.
