Two people are better than one. They can help each other in everything they do.
Suppose someone falls down. Then his friend can help him up. But suppose the man who falls down doesn't have anyone to help him up. Then feel sorry for him!
Or suppose two people lie down together. Then they'll keep warm. But how can one person keep warm alone?
One person could be overpowered. But two people can stand up for themselves. And a rope made out of three cords isn't easily broken.