We love to instantly notify people of the positive activities that happen. When somebody loves the things you do you are instantly notified by a reputation increase. The negative - well, you get to know about it but it's not as instant. We assume those with a negativity need to have a bit of time to evaluate their decision before that choice is made known. We also try to protect people who down vote from being discovered. We
We call it the broken unicorn syndrome: http://www.ted.com/talks/shawn_achor_the_happy_secret_to_better_work.html