Baby A and I have heard so many stories from twins, fraternal and identical, about how much fun it was growing up with a twin. Singletons are right that having a constant companion must sometimes get on your nerves, but most of the time, it's like having your best friend live with you.
I wish I could generalize more. I know one pair of fraternal twin boys who said they were much closer when they were younger. They grew up and grew apart. But I have also heard about "friends of friends" that fought like cats and dogs when young, but actually got much closer as they grew up and matured.
All in all, Baby A and I have always been close. There have been ups and downs, sure, we have had plenty-o-years together. But we are definitely very close twins.
We have had so many singletons tell us how lucky we are to have a built-in best friend. We know, and we definitely appreciate each other.
Tell us your built-in best friend story.