"She was eating my celery."

I realize kids are kids, but why do they do things they know they shouldn't?

Today for lunch my son had chicken nuggets and celery boats. He did a great job eating his chicken as well as the Girl who likes to do what he does and starting eating the celery boats. I was in the other room getting a couple of things done when he says he is done, I say good job and he goes on his way to play in the sunroom. Well suddenly I hear some noise in the kitchen, I see the Girl sitting in his spot chopping on his celery (which I didn't have a problem with) and I notice these orange spots all over one of our chairs. So I ask him what happened, he doesn't respond with words but motions which leads leads to the next question: "Did you spit juice here?" with the answer, "Yes." I immediately send him up to his room as I work on cleaning up the mess. I go up to talk to him and ask why he was spitting juice and the reason was "I was spitting juice at her because she was eating my celery." This could have been almost acceptable except for the fact is he licked all the peanut butter off the celery and didn't want to eat the celery, which I promptly informed him. So he spent some alone time in his room, although I was waiting for the accusation that there was a "second spitter".