If you had siblings, you don't understand the life of an only, in the same way that a man could not really understand the life of a woman or vice versa. A childhood with siblings is different than a childhood without siblings. They each have their pros and cons. Your child's childhood is not going to be like yours. It's not going to have some of the experiences you treasure - but its going to have some things that are benefits that *you* won't truly be able to understand.
My DD will not have any of that . . . thank God. She may have moments of loneliness, but so did I, all the time. She may not have another child to sit with in amusement park rides, but she has me and DH. She will not grow up with anyone calling her names, punching her, or getting her in trouble.