Self-centeredness - people show fakeness in friendship is because they are very self-centered people and don’t care about others showing real friendship.
Narcissism - It is often seen that someone with a narcissistic personality might be a fake friend.
Childhood Trauma - It is also possible that if a person has suffered some kind of trauma as a child; then being a fake friend can be a reciprocal action to that trauma.
Psychopathy - Psychopathy is characterized by a lack of empathy, manipulative behavior, and a disregard for the rights and feelings of others.