In the world of fist to fist combat, there is always a large amount of animosity surrounding it. After all, it takes a certain type of mentality to want to brutally beat down another individual using your fists or other appendages. Even those who get along perfectly will often find themselves wanting to hurt the other person badly, after taking a punch or two.
This happens often in boxing and MMA. However, after a hard session after beating each other down, those who do not have huge egos will often find a sense of comradery and respect for each other. After all, they just engaged in hand to hand combat and showed each others skill and heart.