I am smiley and bubbly. I work in a customer service role. I am dealing with miserable people all day. I'd be mired in the pit of despair if I was not a happy type.
One advantage is that the people who know you, know that when you are unhappy it is serious, not some minor niggle.
I like my job, I like most of the people I work with, my life has the usual ups and downs, but I am just a happy person.