Who are Jacob's best friend in new moon?

Jacob Fights Paul.

The Reason why they fight is because Bella slapped Paul.

(And as said in the movie, Werewolves have temper issues.)

So Bella slapped Paul because she was defending Jacob's feelings.

Then Paul had his temper take over and he turned into his werewolf form.

When Bella was running towards Jacob's House , Jacob jumps from his porch ,

Runs toward Bella, Jumps way over Bella and in the middle of the Air he turns into his Werewolf form!

Then they fight.