Because if your hero keeps bashing mine, my hero starts raging after a while because I'm not GUD.
Thus he deserves to channel his rage into revenge mode.
If he gets bashed by 2+ heroes at the same time, he rages faster.
Just kidding, bro. They should tune gears to slow down revenge building a bit. Ubisoft already said that this is a balancing issue and that it will be addressed :-)