Do turtles need friends?

I have had my turtle for about 15 years now. He has always been an only turtle because I knew I could never handle two of them. He is HUGE and I cannot imagine how big of a tank I would need for 2 of them.
Well I am beginning to feel a bit guilty that he has no friends. I went to our little local zoo and they have the same kind of turtle he is - Florida Cooter. They have at least 4 of those and some red eared sliders.
I am not even sure if they would take him but sometimes I feel I should offer him to them. Maybe he would like to make baby turtles? Or just hang around with someone that looks like him - and not a kitty, fish or a couple of dogs!
What do you guys think? Am I being completely silly? Do turtles even care if they are with another turtle?
I did check and they house them inside in the winter so I don't need to worry about that.
We do have a small pond outback for him. He gets to spend the entire summer outside. Is this enough to keep him happy or does he need more?