Socializing a Dog-Aggressive Dog?

I think my dog is becoming aggressive! I have a 2yr old Pit Bull mix, he's a lovely dog and loves to play. The problem is he's started to fight back when a dog attacks him or initiates a fight with him. A few months ago he would usually run away from confrontation.

After about 3 times of him being attacked (usually with possessive dogs over something as simple as a stick) he's gradually started to become braver and the other day, when he was playing with a dog he knows quite well, they got into a fight. Mine went to get his ball back from this dog who is very possessive with toys, etc. As soon my dog went to get it the other one lunged at him growling and snarling. My dog just went straight back at him!

Thankfully he never does any damage, he just holds them tightly by the neck with his mouth. I managed to get him off, but the other one went straight for his face and now he's got quite a big wound underneath his eye where the tooth went in.

Why has he suddenly started to fight back when before he would just back off and run in the other direction? Thanks for any answers.

By lisa