Word for the behavior of people who complain at everything you do

Solution 1:

If the question is what to call the "behavior" of a nag or naysayer, I would suggest "hyper-critical" to be downright literal, but I would also consider calling this type "oppressive" in the sense that they weigh you down psychologically, or "overbearing" in the sense that they violate your comfort zone to criticize you.

Solution 2:

A fusspot is someone who often complains about unimportant things.

Solution 3:

I prefer the expression carping to hyper-critical as it's more negative. A hyper-critical person could be interpreted as someone who is a perfectionist, acting in a demanding or exacting manner, whereas carping suggests someone who constantly complains, is never satisfied and finds fault in everything you do.

http://www.thefreedictionary.com/hypercritical
http://www.thefreedictionary.com/carping.
http://www.reference.com/example-sentences/carping

"Pedantic and hypercritical, meddlesome and fault-finding, he was a terror to the clerks under him, whom he worried in their work, enforcing the rules rigorously, and arriving himself with such terrible punctuality that not one of them dared to be a moment late."

"I judge a man by his actions with men, much more than by his declarations Godwards—When I find him to be envious, carping, spiteful, hating the successes of others, and complaining that the world has never done enough for him, I am apt to doubt whether his humility before God will atone for his want of manliness."