Like the full moon drawing out luny behavior, somethings are just "known" without ever being "proven." Happy people live longer is one of those. You suspect it, if for no other reason than miserable people often desire an escape from torment. It's the how that stumps folks. Then again, it might be simple:
"A new study led by a Michigan State University business scholar suggests customer-service workers who fake smile throughout the day worsen their mood and withdraw from work, affecting productivity. But workers who smile as a result of cultivating positive thoughts – such as a tropical vacation or a child’s recital – improve their mood and withdraw less."
The link includes such mischief as "Look on the Bright Side" from Monty Python's Life of Brian.
Although attributed to Abraham Lincoln (but something for certain Benjamin Franklin would endorse) is that a person is about as happy as he/she makes up their mind to be. Mortgaging your happiness (or well-being) to someone or something else is a B A D investment. It never pays off. Happy, it appears, is not something that is conjured by external events or others. Happy is a state of Being of appreciation and delight.
I know, I know, those who get it, get it. Those who don't, can't.