Why does my boyfriend ignore me after we fight?

Last night i glanced at his phone and a girl he knows was in his inbox. Later i grabbed his phone to play with it and noticed he deleted the conversation. He thinks i will get mad at him when he talks to his friends that are girls but i tell him over and over and over i dont care. I confronted him and was upset that he deleted it. We argued for a few minutes and he went inside. He hasnt repsonded to any of my calls or texts since. Everytime we fight he does the same thing he wont talk to me and its hard on me to be ignored and i tell him that all the time. Why does he do that?