Complex sentence: Social acceptance is important because many people don't understand mental illness so they don't know how to embrace their friends or other people who have a disease, leaving these people with feelings of not being accepted in groups of friends.
Split into simpler sentences: Social acceptance is important because many people don't understand mental illness so they don't know how to embrace their friends or other people who have a disease. Leaving these people with feelings of not being accepted in groups of friends.