that joke isn't funny anymore

i like jokes. one of the (few) positive things about me is my good sense of humour (even though it gets dark sometimes). someone make a joke out of me, and what the hell, i laugh and join in. when i hear the joke for the second time, it isn't as funny as before - it makes me smile, little more than that. if that someone goes for a third round, then the joke just bores the crap out of me. what, again? now, if the joke goes by the fourth and fifth round, then i get pissed. really pissed. as if that person can't see that the joke isn't funny anymore. that it's getting personal.