When your facial expression doesn't match your inner mood, this can cause friends and acquaintances to check in with you and inquire if you're okay.
And while we can't control how our face looks, we can do some things to make it so our outward expression more closely portrays our inner mental state.
Practice a slight smile or try other subtle actions like changing your posture (a la this TED Talk on the Power Pose by Amy Cuddy).
Even slightly shifting the tone of your voice can go a long way toward creating an outer persona that matches the positive tone we feel inside.