What causes a sociopath to be a sociopath?

The actual thing that causes sociopaths to be sociopaths, Psychopaths to be psychopaths and people with psychosis have psychosis is different in many human beings who are diagnosed or self-diagnosed with it.
The first reason is that you are born with it. or rather born without an important part. There is a very important part of your brain that triggers certain common sense and certain emotions. When this part just doesn't work properly, it causes you to not be able to understand or sense these emotions, such as a mental hurt, and sadness. Anger is usually present however.

Another way is that they're hormones may be messed up beyond repair to where they are so unstable that they cause them to pretty much be on the edge all the time, causing a dangerous sociopath, psychopathy, or someone who has psychosis.

The last reason that I could think of at the time is that someone may of suffered from a traumatic event, causing them to never look at the world the same again, causing trust issues, violent behavior and slight depression. A bad outlook on the world to sum it up totally.

So there are many different factors and reasons, and also theories. it would take a very long time to actually figure out what makes them, them. :)