As a child, I don't think I ever wished or asked or pleaded for a sibling. I was a content only child. It was perhaps a very selfish existence, but it was perfectly wonderful to me. I didn't have to fight over toys or share my mother's love and attention, and when I wanted something, my mom did anything and everything she could to make it happen. I do consider myself to have been spoiled, but not in an ungrateful way...my mom is awesome like that;>
On the other hand, seeing my boys grow up together, I realize there are things I never have and never will experience. They always have someone to play with, whereas I became quite accomplished at entertaining myself with imaginary friends when my friends couldn't play. They have someone with whom to brainstorm, and while that rarely has favorable results from my perspective, I can admire how creative they are with finding ways to get into trouble, bringing me closer to that moment when my head explodes. But mostly, the love they have for one and other is unique when compared to any that I have ever had. My two best friends are definitely the closest I will ever come to having sisters, but I didn't get them until I was in high school, and we never tormented each other like real sisters would have. Even though my boys duke it out over anything and everything, they also comfort each other and take care of each other. M will get J a pair of pants and even help him put them on; J will grab himself a cookie and make sure to get one for M, too. When one of them gets hurt, the other goes to him to pat him on the back and to tell him that it will be okay and even, sometimes, to kiss the boo-boo like Mommy does. To be a witness to this developing bond is an indescribable experience that is one I'm thankful for, even though I never expected it to be a reward of motherhood.
I now understand why people ask parents of only children when they'll give that child a sibling; they know about these things of which I'm only an observer.