What do I say to a girl that wants me to tell her what I like about her and wants to know why I want to be with her. I'm lost for words because I tell her I like being with her but she wants me to tell her more.

Well, what do you like about her? Some girls look for you to give the same answer every time to see if your feelings changed for her.

Be honest with her. I like you because you make me laugh. I like you because you make me feel comfortable. I like you because your intelligent. Etc, etc...