It seems that these letters have evolved from the kanji letters. this: ツ has apparently come from the kanji: 川 that means river. It can look very similar in handwriting if you tilt the lines when writing quickly. I wonder how quickly they made a connection to it looking like a smile when they started using this letter? Also this: シ letter looks like a face too.
@Stonefortune, still, it's a letter, not a smile.
@MARINA_13 Yes I know it is. :)
