First off, we all need to remember that josh does a lot of the writing for paramore. So this song could be about a girl being a decoy.
Secondly, I think its about 2 people in a relationship where one of them likes the other one a lot more and will do anything to please them. The one that's not really into sees what they can gain from it and continues the relationship for its benefits and knows that it'll be easy for them to just up and leave when they are ready.