There are a lot of things that can't be done by only one person. Some things, it's better and more fun to be done when there's two of you, or even when you're in a group.
Like for example, going shopping is better when you're with your friends. You get to see different sides and opinions, and looking for clothes are easier, since there are more than two eyes looking for a cool piece of clothing.
Playing chess or checkers, or any other game, is something that can be done by two people. Although, there are times when two people work together, to beat only one opponent.
There are a lot of other things that takes two.
Like being in a relationship.
But unfortunately, it only takes one to ruin it all.