I have a male dog,my friends have two females.My male picked the bigger and older female that has been fixed.Why?

Don't even listen to paul. In my opinion, he's more of a troll and a creep. Dogs are smart. There are many different reasons why a dog will chose another dog. It can actually be for friendship. If he's not trying to dominate or hump her then he feels good being with her. They know when the female is fixed because they can smell hormone scents.

Are you trying to BYB your dog? Why?

He is trying to dominate her & is not trying to hump her.

Doesn't sound like you know much about what is going on. Don't try to breed him, get him neutered so he cannot reproduce. BYBers are responsible for ALL the dogs in pounds, shelters & rescues. You will not ever find a good quality dog in any of these places, unless it is a breed specific rescue.

http://www.k9web.com/dog-faqs/breeding.html

He needs to be neutered & not bred.

He didn't "pick' her, he was trying to dominate her. You need to look up what that means as far as dogs go since your question tells one and all you don't know nearly anything about dogs.

Because you have no control over your dog and failed to train it.

just cuz. its not like he knows she lacks a uterus and ovaries. its not like hes into birth control.

after reading your other questions ....the answer is YOU ARE A TROLL

What do you mean - picked....