Why do girls act the way they do?

This girl and I have started dating, for about a month, and I know she is interested in me because I asked and her friends told me. We only have the summer to date and we both have said that we don't want anything too serious but I do like this girl; probably more than I want to admit. What I don't get is that she acts a little distant. We hang out, go on dates and yet she just acts like we are friends. We have even kissed and made out but she doesn't really let me get close to her. I am also the one who always initiates conversation through texts and suggest hanging out, with only one or two exceptions. Please help I am just confused and want to make sure we can have a fun summer without any walls or barriers between us.