Maybe you see something in her, perhaps her fun side. As plain as she may be you see something completely opposite and just can't seem to put your finger on it. You can try to bring out her fun side, it'll take time but who knows. It could be worth it. She probably wants to show you that side of her but isn't really sure how. Or, ORR.. If you're the outgoing type and since the women you go for are all boring, they balance you out. Attraction is also about balance.
Maybe subconsciously you want girls you think other men don't want so that you don't have to worry, about other guys swooping in to try and steal her. I only say that, because you yourself find her plain and boring. Most people go for someone that excites them.