Humans, including kids, are inherently social creatures, and are happiest when communing with their fellow creatures (including pets, but mostly other humans). Kids are most happy when they and their friends are playing together, and my personal belief is that they are happiest when helping other people together (so much of my childhood play was around pretending being a hero, sacrificing and risking my life for the sake of others - what if that was channeled to real world aid).