here 
There is one simple thing that has been medically proven to improve the immune system and increase your positive brain chemistry. That one simple thing is smiling.
I’m not talking about standing around like a moron in a daze, but actually consciously changing your mood by thinking of something to smile about. Maybe it’s just the Californian in me, but I’ve found that if you have nothing to smile about, smile about that. See? Better! The most important reason to smile, is that like reducing your intake of animal food products, smiling has been medically proven to improve the immune system and increase your positive brain chemistry through the simple act of smiling. But that’s not the only reason to smile... there are many benefits, including but not limited to the following:
The Attraction Factor: Nobody like a grumpus face, instead people are drawn to people who smile. Walking around frowning, scowling and grimacing simply pushes people away. Obama’s consistently fake smile has earned him millions of fans and friends, check it out:
So if you notice a lack of people noticing you, smile more. Smiling also changes your mood. Dr. Dale Anderson, who wrote Smile, Laugh, Connect: The Key to Increased Morale outlines how endorphin levels are raised higher through a range of fun activities like laughing, smiling, eating exercising, cheering, singing, listening to music, creative visualizing, camaraderie and romance. Seriously, don’t those things make you smile and feel better?
Like a yawn, smiling is contagious, a smiler lightens up the room, changing everyone’s mood by making them feel happier. A smiling person brings happiness with them. Smiling also prevents us from looking tired, worn down or overwhelmed. When you feel stressed, simply putting a smile on your face changes your mood, making you feel better. Reducing stress means your immune function is more relaxed, functioning better by sending beneficial chemicals throughout your brain and body. This is because when you smile there is a measurable reduction in blood pressure and a release of Endorphins and Serotonin, chemicals which induce good feelings. Smiling is a natural drug.
Physically, smiling lifts the face, creating a youthful look. Smiling through out the day is better than a face lift and less expensive. It leaves an impression of success, making you appear more confident, more likely to be promoted, and more likely to be approached. Smile at meetings and appointments. People will react to you more positively.
Here’s a test: Think of something negative while smiling. It is difficult to think of negative things while smiling. When your mouth is turned up or your mouth is turned down, so goes your day. Pause now. Decide to make this moment a great one - smile. Then, make the next and the next moment a great one until you have a great day.
At the very least, smile a lot to keep your enemies guessing!