What are reasons you block someone?

I have noticed an increase in the number of people who have blocked me. I am puzzled because I don't recognize the names and wonder what I did that made them block me. I try my best to be friendly and not throw negative comments around. As I have read some of the questions and comments I have noticed more deleted messages with replies. If I click the tagged name it shows that I have been blocked by them. I have never found a good enough reason to block someone.

This leads me to the question of what causes you to block someone?