Why Poke the Bear?

Granted I would never poke an actual bear but this is something I wonder of my children sometimes.  The bear in this instance would be each other.

When one of the kids is not happy about some tiny little unimportant issue, why must the other child poke the bear.  I mean the Girl can already be pissed at me for just trying to help her sustain life by offering her food and the Boy has to come up and get in her face, say something to her, or just be too close.  She will make it known that she doesn't want you to look at her or talk to her but the Boy insists on talking to her or looking at her.

When the Boy is playing with his Legos, or down in the basement playing with his trains, the girl must go and grab something from him or bug him to no end.  He gets mad and reacts and then she cries because she poked the bear and he didn't like it.

The issue is they don't learn anything other than the other gets mad, and then they get sent to timeout but that is no deterrent to them not doing the same thing two minutes later.  I wonder if they would learn if the poke a real live bear with a stick or something, knowing my luck the bear would play nicely with them increasing the madness of the situation.