It makes me wish we could have given our daughter another sibling. There are challenges in raising children, and there are special challenges in raising single children.
On the Mark, your daughter knows she is loved, I'm sure. Being an only child, she may not having a sibling to play with, but she has advantages too. She gets more of your attention and financial resources.
It's great watching your children getting along. I remember my children would sit there playing together, laughing one moment and then the next they would yell at each other.
On the Mark, I have a nephew who is in his late teens and he seems to have come through his childhood as an only child fine. They make up for not having a sibling by having close friends.