Why do people rock back and forth?

i think that some people rock back and forth because they feel insecure and lonley. maybe if some people tried to comfort them they wouldn't do that anymore! My boyfriend and I have been dating for about 4 years and lived together a year. We just moved in together about a year and a half ago. I remember within the 4 years of us dating he rocked but I never understood it. Sometimes when I was upset with him while we were laying in bed he would rock and I would think he was doing it to make me mad cause I can't stand movement in the bed. I have to lay still. Now that we've moved in together a year ago he rocks every night, when hes thinking or when he's upset. It's very odd to me that he does this and I've read peoples answers about rocking and it is a common thing with people. These stories have really helped me understand what he's going through. I would get mad at him when were watching tv together and he would leave to go rock in the bedroom...but now i know that i can't get mad because it's a problem with him and he cannot help what he does. Thanks everyone. If anyone finds an Answer to why or a clue why... please let me know.