Other than feeling bad for the poor girl, how is it that in manga people are portrayed to fall in love with someone they don't know (and be in love for a long time) just for a single act of kindness?

I think at this point it's just another staple of Japanese media, like pervy old man, manzai (straight man, funny man) comedy or clueless main character. Besides, is it really that different from love at first sight that's so popular in western stories?

Fair point.