It's an unconscious and, in my mind, a perfectly normal reaction to a humorous recollection, and I'm quite sure it happens to everyone at least once in their lifetime (unless of course you are my old music teacher who probably thought any expression/form/sign of happiness was a sign of blasphemy that should be "plucked out of the repulsive child until it is non -existent").
And yet, people will look at you, and take another long hard look at yourself, and will then decide that you're one of Mother Nature's experiments gone all wrong. They will then proceed to skiddadle/scatter/flee as far away as possible from you. Unless of course they are Alpha Morons, in which case they'd think you're smiling at them because they are "so hot" and hence there would be no getting rid of them.This has happened to me on a number of occasions, and my smile usually tends to widen when I realize what a reaction I'm causing (to normal people, not Alpha Morons)! I also tend to get a two-seater on the bus/train all to myself as a result, so I must say it is extremely advantageous to smile and continue to smile throughout your journey. Practice is of course required. Stamina is also required, or those cheeks will ache like crazy after a few minutes. And the more genuinely loony you can make that smile, the better chances you have of a trouble-free, non jam-packed journey. Insanity is just a stone's throw away, my friends! :-)