Something that happens to a person's thinking can lead to paranoia. You may believe that your friends no longer like you, but it's just that your paranoia makes you suspicious of everyone.
Paranoia is from the Greek para-, "beyond, irregular," and noos, "mind." It is an irregularity of the mind, a mental disorder that distorts a person's perception of what is real. Someone with paranoia may believe that other people are out to get them or are watching them all the time. Doctors who treat paranoia look for causes, and while patients can develop the disorder gradually and sometimes due to life circumstances, such as abuse or neglect, the use and abuse of drugs can also cause paranoia.