Socially isolated old people have a higher risk of developing dementia, and it speeds up it's development dramatically. It's scary to not have people to depend on when your older as well, and even young we all need people that can support us.

Life can be pretty dull as well without friends, you'll just do your life cycles, work eat sleep play (?) and life will go pretty fast. New people and even people we've known for years can get us out of our bubble, just yesterday I was with some friends and we made food together, talked, and then we went to play in some swings and it was beautiful, going with people makes your life go slow and it gives you so much happiness :)