i smile to hide the pain
You fake a smile because your soul is screaming, people think you're a weirdo who's always happy, but the more nonsense you get, the bigger your grin gets until you lose it over something tiny.
I smiled through the whole day, then exploded over a messed-up lunch.
He grinned like a fool, then yelled at the vending machine.
She laughed at my failure, then cried in the restroom.