Thanks for the reply!
He gets angry at things when he really shouldn't.
For example, the other day while I was at his house, he went to take out the garbage. I heard him saying "I'm bringing the keys with me". So I knew he had the keys with him and followed him outside the house.
Then, the door closed behind my back because of the wind.
He turned around and looked at me saying "And now? How are we supposed to enter the house?" So I told him that before exiting I heard him say he had the keys with him, I wouldn't risk going out without making sure the keys are with us. And he got so irritated because "And what if I didn't have them with me?"
But you have them with you! It's not like we're stuck outside, we have the solution to this problem!! So what's the point in getting angry at this, really? I don't understand. I feel like a normal reaction would have been "It's fine, don't worry, I have the keys with me" and just laugh it off.
To me his anger seemed very unnecessary. And when I told him what I thought, he said to me "It is normal that things like that piss me off, others would be screaming at this instead". Nice...