Because you have the responsibility of ensuring that another living creature is cared for, has a home, enough food, medical care when necessary, and above all feels loved and wanted. In return, they give you love, and trust you to be there when they need you, and to ensure that their life is structured to keep them safe. Their dependence and appreciation of your actions makes you feel like a person who is part of the universe, you are needed, and reliable enough to earn another creature’s love.