Flame in Champion's selection

Why people starts flaming in champion's selection? Just because i ban and they starts say "omg bronze bans" "noob bans" "idiot" etc etc. I just don't want good champions to be banned. "Ban Darius ban Darius!" but why?? if you think Darius is Op i don't ban him so you can pick him..if you don't pick him and then enemies pick him it's not my fault.