If two people sleep together, they will be warm. But a person sleeping alone will not be warm.
If one person falls, the other person can reach out to help. But those who are alone when they fall have no one to help them.
An enemy might be able to defeat one person, but two people can stand back-to-back to defend each other. And three people are even stronger. They are like a rope that has three parts wrapped together—it is very hard to break.