There was once a moment when I questioned myself the reason behind hiding my hand inside my jacket pockets. Not trouser pocket, jacket pocket. For some reason I feel weird to pocket my hand there but jacket is another thing.
First reason would be that my environment for that time was cold so every time weather grows cold, I think it is only natural to hide my hands inside jacket pockets. Looking for warmness, right?
Second, looking good all the time. For example, well you know all of the examples. Every good-looking, cool, mysterious, and awesome character always ends up with either nice suit with hands in the leg pockets or bringing very big and long broadsword along with long sleeveless shirt and long black trouser completed with black pair of shoes or both. So, you may try to be like one of them.
Third, no need to touch body part of somebody else. For example, if you accidentally touch people, especially a girl or immature woman in the sidewalk, you may get some problem with their hypersensitive skin of some body parts and if that happens, you are almost certainly going to be called pervert. Trust me, you don’t want that to happen. The other is if you happen to bump a big guy and his companions. I guess you are going to have a big fight ahead and you will definitely lose unless you have already mastered some martial arts. Different story will come if you nudge them with your shoulder or elbow, events like those are not likely to happen as long as you say sorry and keep walking.
Fourth, keep people away. There is a stereotype that if one hides their hands inside the pocket, either jacket or pants, they are surely worth not invited to talk and this is the best thing ever to you who prefer being alone and think of more important things to getting loud inside the crowd and wasting your time in talks you never begin in first time. So,what I am saying is if you sometimes want to alone for a moment, put your hands inside your pocket and slowly other people will ignore your presence.
Fifth and this is reason of my own making, my hands have no use at that time. I mean that there are always events, moments, where I have to talk with someone or audience of which my hands have nothing to do. So, instead of swinging my hand around for some pointless activities, I put one of them inside and the other out because at times I have to hold my microphone.
Hands on deck? I prefer inside my trouser/jacket pockets. Good day.