why are people mean to others?

There are number of reasons why people could act mean towards someone else.

I would argue, that the main reasons are :

Because of the unpleasant feelings people get when they are reminded of their anxieties some of them take extra measures in order to prevent themselves from encountering these situations that remind them of their anxieties.

Those who harshly criticize you or judge you for no reason, might be the people who feel inferior, and who try to safeguard themselves by devaluing others in order to feel superior.

We are a reflection of our own environment....

Just a thought-