Living in the moment

Why don’t we live in the moment and enjoy life? Is there an unwritten rule that we should not be happy for too long. As kids we lived everyday as if its our last we never stayed sad or upset for too long, we'd have a cry and then just move on..But somewhere along the road of life we just stop having fun and we’ve become stressed, depressed and traumatized. Can we not let out our inner child to have fun and to just relax and give gratitude to life and everyone around us.