The problem is, it's who you go for.
If you go for people who are idiots, they only want an idiot to accompany them. If you go for someone who's skinny blonde with massive whammers, who spends every night getting hammered on free drinks from guys who want a quick one, you're going to find out why she only has one nighters.
There's no point in trying to convert someone, you need to find the right person. I know 2 or 3 girls who are perfect girlfriend material, dedicated, pretty and lovely, but they wouldn't go anywhere near someone who always expects they're up to something when they aren't. It's swings and roundabouts, if you always expect the worst of people, they won't give you the time of day. Who would?
If someone is ALWAYS negative about EVERYTHING, why do I want my mood bought down by them being a sap?