Why do they...

Why do they pick on each other so much, I just don't understand it.  You would think that they would get tired of it by now. I know that they are with each other everyday... but come on REALLY! Sometimes I want to just start picking on them, just to see how they like it. Knowing them, they would think that was funny.  I can not wait for AJ (my 18 year old) to get a job.  He is the one that picks on the others the most.  Hopefully him being out of the house more will mean more quiet times.  I think because he is the oldest he doesn't get picked on as much as all the others.  I should have a "Pick on AJ Day".  Maybe that will get his butt moving on getting a job.