Psychology has a lot to say about people who repeatedly find themselves in these types of relationships or ill-defined situations. One of the most common reasons for ending up in an undesired situation is that you're not making your intentions clear to the other person from the very beginning. Not trying confuse their desires or try and read things into it normally the simplest explanation is the right one. It's important to be able to identify what it is internally that's drawing you to people who aren't going to commit to you rather than looking to change them.