Siblings-why must you try so hard to irritate one another?

Only children are notoriously and stereotypically self centered and full of emotional issues. But honestly, why do some of us feel a desire to have more than one child? I mean, I love my kids and wouldn't trade them for the world, but what was I thinking, having more than one? Did I have some fantasy that the constant bickering that can occur between siblings would not happen in my household? Did I somehow believe that my children would be from some perfect universe where they never insulted or hurt or angered one another? Honestly, I think I didn't even contemplate it. When I decided to have more than one child, I do not remember even entertaining thoughts about how they would be to each other. Except when I was pregnant with one and tried to educate the older one(s) on what to expect with a baby in the house and how I may need their help with some things, I didn't spend time considering how the children would get along. When, as a mother of 3, I met my current husband, who had 2 of his own already, I did not think much about those children not forming bonds or relationships. We gave them time to spend together and helped them work out issues when they arose and hoped for the best outcome, I suppose.

Here I am, eight children later, all but my youngest two out of the home during the school days, being driven to the brink of insanity by kids who aren't old enough to tie their own shoes. I am perpetually astonished by my toddler's desire and ability to annoy my pre-schooler. Its like she lives for it. Is he playing with legos or dinosaurs? She wants to interrupt, ruin or otherwise interfere or take over the play session. Is he drawing or coloring or making a craft? She wants to take all of his supplies and run to the other room, throwing them about along the way. She doesn't seem to have a desire to copy what he is doing, leading her to this unending need to be involved in it all, she just seems to want to mess it up. 

He's not much better. Whatever she is doing, if he's not already so heavily involved in his own play that he doesn't notice her, he will go over and climb onto her or knock the toys out of her hand. How do most of us grow up to be decent adults after having started out having such intense focus to ruin others' happiness? 

Siblings require a lot of patience. Patience I didn't realize I possessed until I had more than one child. When I added a second child, it changed everything. When I added a third child and therefore had outnumbered myself as far as ratio of hands to children, it changed things even more. Adding more children to the mix after that didn't really change my parenting that much, but having these youngest two and especially tolerating them within the constraints of fighting an often fatal disease like breast cancer, has certainly tested my ability to stay sane. I'm not really sure that I am sane most days. There are times when I have absolutely no patience for the constant chatter of my four year old or the constant "mommy, mommy" of my nearly two year old. There's barely ever a day I have enough patience to tolerate the non-stop urges my youngest children have to make each other's lives a living hell. All I can do is try my best to teach them not to find joy in getting on each other's nerves and how not to let the need to compare themselves consume everything they do.