Rejected

 Why do humans are afraid of getting rejected?

one reason can be, that humans are social animals and need to socialize with other humans for survival. 

We are programmed to survive, and when we are surrounded by people we get a sense of security and hence longer life. that is exactly why people are afraid of being alone. 

As social animal isolation can be extremely painful and can even cause a brain haemorrhage, and even death. 

Infact according to some researchers pain of rejection is equal to any physical pain.