How do you deal with an overly defensive spouse or person?

It's been the same thing with my fiancee from day one, and I've learned to deal with it, but sometimes it's too much. The problem I have is that he is overly defensive about everything. He overreacts over the smallest most insignificant things and I could never understand why. He use to be worse but I still don't know what to do in these situations and it's getting to me now. Example, a regular noise outside would cause him to jump and gasp. We've never watched a scary movie together not once in 4 years he can't take it. When we first met I was having a bad day and told him I was feeling sad, he replied in a rude and defensive way "So what, you're saying you don't want to be with me?" This is when I first noticed he had a problem. Today he was going somewhere and he came back about 20 minutes later again because he forgot something and all I said was "You're taking too long" He was going to the park to play basketball, but if he takes too long the courts fill up and he gets home late. He turned around angrily and said "I had to cook for you!" The funny thing is that he didn't cook. He never cooks, I cook everyday. Today was the first time I have ever asked him if he could help prepare something to eat because I was suppose to be studying for a big exam. AND, he didn't cook, he boiled some hot dogs, but that's beside the point. My point is I can give example over example of how he overreacts about everything and is always defensive. Like one day he was trying to plug a DVD player and some other equipment to the TV and all I asked was "Do you know how to do it?" He overreacted and said "FINE, do YOU want to do it??" I was like oh my god it was just an honest simple question and he's like "Well you're saying I can't do it!!" My god, he overreacts for everything. When I talk, I'm being normal but he says it's the way I say it. But it's not me because when I ask, I'm calm and really just asking a question or making a comment and he gets all crazy about it. Even at the grocery store he was really rude to the bag boy the other day and he said that the checkout stand guy was being a jerk but he wasn't! Why is this man so overly defensive and how can I get through to him that he's overreacting all the time? If I try explaining this to him he insists it's me. I know it's not me because one day my mom asked him a simple question when he was at her house. She asked if he had seen some soap or something. He got defensive and said he didn't take it but would replace it for her. He was already thinking she was accusing him of stealing it! Even my mom couldn't understand him. It's just too much. Again this is just a few examples out of hundreds. Why is he like this?