why would a colleague become aggressive during an argument we had for fun?

summary: Exchange words, he tried intimidating me, i didn’t give in, he hit me (his palm with motorcycle gloves on the back of my neck) i returned the favor, (my knuckles into his ribs) he wanted to solve it outside work with our fists[i nvr expected to hear him say that] (first time it happened i was respectful and returned the same amount of force and verbal exchange) whats your opinion?