The girls who generally do like to give head are usually just trying to keep their man happy- It's not something that really gives us any pleasure, it's just nice knowing your guy is satisfied with you in general.
Girls who don't like giving head usually don't care if the guy likes them or not, they just move on to the next guy if she feels she's been 'used' because these types of girls tend to always have an issue with something and are very difficult.
I've heard some girls are scared to do it in case they do it wrong -and don't know whether they have to already be hard or not for it to 'work'....