It's a never-ending line of people attacking one another when not even provoked by them that causes the next generations to be affected by the confusion of whether to get revenge or forgive and forget.
I'm sure something will happen in this century that will cause people to want revenge in the next one.
A lot of it is just politics, too. Leaders shout "Revenge!" and the public is motivated. But the men behind it all usually aren't so foolish...exceptions, I suppose, would be certain religious extremists.