Why do people discriminate against others based on their race, sexual orientation, nationality, or other reasons?

Prejudice is just that: "pre" and "judge".

That, right there, shows that prejudice is lack of personal examination. It is taking what you were taught, by whomever taught it, and not examining it to assess, for yourself, if it is correct.

It is easier that way. If you do not examine your teachings, you are not going to find that they were wrong. Also, you do not have to put forth the effort to think for yourself. Furthermore, in many cases of prejudice, the person being targeted by prejudice is targeted by many people. Hence, going along with the crowd brings the "bennies" of fitting in.

It is simply herd mentality.

Prejudice and stereotypes are easy, you see. It, without any dissection and analysis, puts a huge definition on a person. Unfortunately, most stereotypes are wholly inaccurate.

To me, bigotry is one of the worst traits a human can have.

I also believe that the truly mean and cruel amongst us tend to be bigots. It is an easy excuse for them to perpetrate their hate and even crimes against another.

Now, there is personal experience that shapes our beliefs about certain groups of people.

I lived in Gangland Central for a bit and I have had to fight more than once, I have been shot at, etc. I, for example, would not walk up to a group of gang toughs, or people that I assessed as gang toughs, and say "Hi there!" I know from life experience that this is not a good idea. However, if I came across someone who I identified as a gang tough and they were nice to me, I would reevaluate my belief about that person. I also am not going to treat someone I identify as a gang tough poorly unless they have shown me reason. When I lived in Gangland Central, I evaluated each person and wound up with careful friendships with some of them. Yes, I was careful because that is how to stay alive.

There is the difference between a bigot and someone who is not. I do not look at anyone and mutter a string of curses ending in "f*cking [enter label here]". If someone proves, by behaviour either good or not bad, I see that.

I do not care what a bigot is bigoted against. I consider bigotry to be a personal flaw. If, however, a bigot shows that s/he is able to make up his or her own mind by evaluating their beliefs and seeing through their prejudice, I am just fine with them.