Why are people so mean ?

Most of the time it's because other people let them get away with it.  You can control how you respond if other people treat you badly.  Don't allow anyone to treat you badly.

People are "mean", if you want to call it that, because they are afraid. Fear is more than just nurture, more than the sick twisted feeling being rammed down our throats since childhood, it's biological, a constant humm reminding you that at the end of the day, you're on your own.

This is a cold, rational view, I know. But I honestly think it all comes down to the basics. People are mean because we are still (but nevertheless temporarily) animals.

I do believe that they are "afraid" But that is a symptom of something much deeper; Ego. The Ego is often the inverse to what's subconsciously true. Basically, people tend to overcompensate for what they secretly fear or think is the truth.

Examples: People that go around harassing the overweight are often afraid to be overweight, or think that they are overweight. Likewise, people that say others are "stupid" are either afraid of, or think that they are "stupid" themselves. I believe that even criminals are such, because they have low self-worth and feel incompetent.

ROFL!: I just saw your slogan, TalSH. One of my favorite movies. Love Kevin Smith.

"Most of the time, I'm just talking out of my ass - or sticking my hand in it." 

"f***ers think that just cause a guy reads comics that he can't start some sh*t" 

They're not really mean, they just want to be loved

While I think it is certainly possible, I don't believe being 'mean' stems from either fear or a need to be loved.  In fact, studies have shown that generally children who could be considered bullies tend to be otherwise well adjusted with high self-esteem.  Henry Murray believed that we are subject to needs which range from the simple (need for food, water, etc.) to more complex needs (need for affiliation, need for power, need for achievement, etc.) which serve to support the more basic ones.  I believe that being 'mean' is the result of an attempt to gain power over others, to see them as being subordinate to one's self.  This is just some stuff I read in a psych class I took recently so I'm by no means knowledgeable on this subject, but it makes sense to me.