The evolutionary reason is survival. When you were ostracized from your group, or attacked by others in the group, for tens of thousands of years, that meant certain death.

Now it doesn't, and our emotional evolution hasn't caught up to our society or our intellectual sophistication. You don't have to respond to conflict at all, in 99% of cases. There is no benefit to winning. If someone cuts you off in traffic, getting mad and honking causes more pain for you AND for the other person. The most beneficial thing is to not even react other than to prevent a crash.