Why doesn't he do what I want?

Me and my boyfriend have been dating for a few months now and I’ve started to notice something; we always seem to be doing what he wants to. When ever we’re bored he’ll usually decide that we’re going to go look at cars, which I’m totally cool with cause I know that cars are his passion. Then when ever we decide to hang out with someone, instead of just being alone together, we always hang out with his friends. I’ve been pretty ok with these types of things. But today me and one of my friends are wanting to go to the pool and play some voley ball. I asked my boyfriend if he wanted to come along. First, he asked how much it would cost. I told him nothing, since I had a free guest pass. Then he just said “Uhh… ok.” See, I tend to take that as a sign that he didn’t want to go. So, even though I really wanted him to go with me, I told him that he didn’t have to, that it was just an offer. Then he was just like, “Well my neck’s really sore babe.” I’m trying really hard not to let this get to me. But it kind of sucks (and hurts just a little) that he isn’t willing to go with me. I mean, I know his neck hurts… but I went through a car junk yard with him a few weeks ago, even though I had a pretty bad cold, because I knew he really wanted to do it. How do I deal with this?