Do you know what a boomerang is? It's that toy you throw and it comes back to your hand. Life is just like a boomerang. If you throw "bad things" (meanness, shoving, gossip) at your friends, sadness comes back and hits you. You end up badly. But if you throw "good things" (lending your toy, helping the friend who fell), happiness comes back to you!
God made the world this way: whoever plants kindness, harvests joy. You don't need to wait to go to heaven to be happy, we can be happy right now by helping people!