According to Dr. Murray Grossan and Dr. Sivan Finkel, the act of smiling makes you more happy. Signals are set off in your brain that trigger neuropeptide release, fighting off stress. These signals also release neurotransmitters including dopamine, endorphins and serotonin, lifting your mood. So, basically, smiling tricks your brain into thinking you are happy by releasing neurotransmitters. Additionally, Multiple studies have found that smiling leads to longevity and lowers blood pressure in stressful situations. 

Smiling can also improve the moods of those around you. When you see someone smile, your brain automatically wants to make you smile, too. And then when you smile, you get all the benefits mentioned above. So, smiling at someone else helps them too!