A dumb smile is, in appearance, an illogically big smile that one makes when a person they love (for me, my boyfriend. i love him deeply) acts in a sweet, cute, hot, sexy, affectionate or/and kind way (from my boyfriend, all of the above way too many times a day).
It also indicates a deep, intense and irrational state of joy (yes it is, i can't put words on how happy my bf makes me)
This smile can mostly be caused by a partner, a crush, a celebrity or a friend. A stranger's actions being a cause of it is a possibility, but a quite rare one. (In my case, only my boyfriend makes me smile dumbly and i only have eyes for him.)
him: "...my softie cutie wifey that I luvvvvvvvvv <33333"
me: *extremely dumb smile*