Why is there that one person at your job that you CAN'T STAND!??!

I cant stand it when I come to work, everything is all good, everyone's in good spirit..... and then... there's that cancer! That dude who should've been born with a vagina! Always bi**h*ng and complaining about what everyone does at work. How many of us have these kind of people at work that we just can't stand? That guy/girl that knows how to to bring the group morality down as a whole!?