Why does my cousin act this way?

Ok so I'm 19 and my (male) cousin is 23. We've grown up together pretty close; our mothers are sisters and kinda like best friends. He was the only child, and I had a younger brother. When we were younger he would always play with me and carry me around; kinda treating me like his baby sister. Well when I turned about 6 or 7 my parents moved us to another state and we only would visit about once every other month now. Our family is still close, but the visits decreased and so we don't exactly "know" each other as well. Especially when we became teenagers, we were in different worlds considering he is older than me. I started dating at 16, and every time I would visit he would ask questions about my significant other with bitterness. Then behind my back tell my Mimi "she's just too young be dating" well I got serious with this one guy when I turned 18, he was actually a friend of his. He hated it. He would all the time make comments about him, tell my Mimi he thought he was "no good" and said he wished I would find someone better. Constantly nagging me about the situation. And when we were together he was all happy and hanging out with me, but the minute my boyfriend showed up he was quiet and acted like he was pissed. I asked him "what's your problem. I'm 18 and I love this guy" he said "I just don't like him, he's weird". Months down the road we ended up breaking up. He was happy about it. Well 4 months later I'm with this other guy. Someone he doesn't know. And I really love him. I haven't visiting since Christmas, so I finally did this past weekend, with my wonderful boyfriend. We met up with my cousins and went fishing, same old scenario with my cousin. He ignored him, didn't even give try to bond. When I got home I sent a text to all my family thanking them for letting us visit. He responded "he's cool....I guess" What's my cousins deal? I don't get it.