Because without the existence of things other than yourself, you cannot know your own shape. Yes, you know your own shape through seeing how other people are shaped. By looking at the barrier between you and other people, you visualize your own shape. You cannot see yourself unless there are other people. I can exist because there are other people around! If I'm alone, I'll always be alone, no matter how far I'll go with it! All the world will be just me. By recognizing the differences between yourself and others, you shape who you are. The very first other person is your mother. Your mother is a person separate from you.