Fighting has a huge signalling component: when viewed in isolation, a fight might be trivially, obviously, a net negative for both participants. However, either or both! participants might in the future win more concessions for their willingness to fight alone than the loss of the fight. As humans are adaption executers, a certain willingness to fight, to seek revenge, etc. is pretty common. At least, this seems to be the dominant theory and sensible to me.