I was like Phyllis in that I went through a period of time in my life to where I didn't smile much because of personal turmoil. I was a constant worrier and because of it I seldom cracked a smile. I'll never forget what a woman said to me one day when I was walking through a mall with my son. She asked me if I wanted a cosmetic makeover. I politely declined. She looked to her coworker, shaking her head and said to me, "Such a serious look for a pretty lady." "Smile."
I'm serious in nature but life got a whole lot better for me and I'm happy to report that I try to smile when I genuinely feel like smiling - which is quite a lot now. Like Lisa said, it's true, when you smile you feel better internally and your whole mood is lifted. But I spend a lot of time at home alone and I feel kind of silly walking around the house smiling while I'm alone, but I suppose I could try out the theory to see if I feel more perky.
Like Jase, my son is a big smiler (and joke teller too) and I noticed that people are drawn to him because of his happy demeanor. Just to be around his happy personality uplifts you and puts you in a good mood.