lots of people think "i don't start fights, but i do fight back".
but there are lots of fights between people who BOTH think like that. so who started it? a misunderstanding started the fight, then someone started fighting "back" in response to a misunderstanding.
the "fight back" type people FREQUENTLY end up being the first one to be mean and fighty. cuz they routinely think the other guy already started a fight, when he hadn't. so they end up initiating pretty often, sorta by accident, but they deserve a lot of blame here.
people also misunderstand how severe an attack was. like it could be someone really was attacking you, but only weakly, and you misunderstand it as strongly. the misunderstandings can be about degree of attacking, not just whether or not they were attacking.
if you wanna be a better, nicer person, you gotta at least REALLY check for misunderstandings and make sure things are REALLY REALLY EXTRA SUPER CLEAR before you think someone is attacking you and fight back.
also, even if you were correct that someone is attacking you, it's often best not to fight back anyway. lots of attacks can be ignored, like if someone insults you what are you going to gain by fighting back? it'd usually be better to drop the matter.
note i'm not talking about like a guy coming at you with a knife. if he's shooting a gun at you, go ahead and fight back!
but if you think someone insults you, then it could easily be a misunderstanding. same with mean jokes, social slights & snubbing, voice tones, facial expressions, and "unconstructive" criticism. this stuff is more subtle and much easier to misunderstand than physical violence.