Why did he wait three years to make a move?

So in short we met three years ago at school and became study buddies but as soon as we got our AAs we went to different universities (ten minutes away) and only talked via text because school was our focus. He (D) would text me good morning and good night for two years, till two months ago when D wanted to hang out in person. I agreed and then D flaked so I stopped replying to D's text. Then just this past friday I get a random text from one of his friends (C) and I'm forced to text D to ask who this person texting me was. D replied that he wanted to hook me up with his friend (C) and we should go out sat. So we did, D, C, myself and my sister. Next thing I know me and D are hooking up. After we you know, D went to the restroom and I bailed without saying goodbye. D then started texting me sorry for acting a fool and he doesn't want things to be weird between us. I told D that I was into him. But he didn't really respond. Then tues D woke me up with the usual good morning text and said he wanted to hangout again. I have yet to respond. Im not one to date friends but I reallly do care for him. I just don't know why D waited three years to make a move? Should I persue a relationship or was it just a booty call? I'm so confused.