Me and my boyfriend were making out it was getting really hot and intense and I had nothing to grab onto so I started clawing at his sides, when I started to do that he started to kiss me harder and pinned himself hard against me. Why did he act like that?

When you kiss someone, what your hands tells the other person how you are liking it and how intense you want it to be. That's why when you see someone just staying still when they kiss, you get the feeling that they aren't really liking the kiss. This frequently happens in Korean dramas during a kiss scene and makes many viewers frustrated (if you have no idea what I'm talking about, don't worry about it). But in your case, your hands were showing signs of aggression and your boyfriend took it as you wanting to be more passionate and aggressive with the kiss. It's pretty natural that your boyfriend took it this way so it's not something you need to worry about unless you don't like it. If you don't like that, then you need to be aware of what your hands are doing and make sure they are more passive and gentle so you don't send him the wrong message. This will then reflect the kind of kiss you'll receive from your boyfriend. I don't know the real scientific explanation that goes behind this but I know this from personal and other people's experience. Hope this helps!