If a loved one of mine is getting attacked, beat up to death, or anything along those lines, I'm certainly not standing by to watch. I'll do my best to break it up, but if the other person isn't showing any signs of letting up, then I may have to fight as well. In any other circumstance, jumping isn't okay. I've seen people jump in fights just because they felt hyped up and that adrenaline kicked in, and some just do it for the ki, that "haha, we got you good" mentality. Reasons like those definitely aren't acceptable.