Ponyo first met Sosuke when she was a humanoid fish. When he cut himself saving her from the bottle she was trapped in, she responded in kind by healing his thumb by licking the wound. This in turn became her first step to not only be human, but also a great friend of Sosuke. When she does become human for the first time, she spends most of it playing with Sosuke and helping him and Lisa out during the storm she accidentally unleashed.
One of Ponyo's first words to Sosuke was "Ponyo loves Sosuke." And several hints around the movie showed it was that kind of love. She got jealous of Kumiko and spat water on her for insulting her. Rather than wait for Sosuke to kiss the bubble as part of the Spell Granmamere put on her, she jumped at him and kissed him through the bubble.