We don't know for sure. There have been, however, several hints! In episode 275 'Gotta Catch Ya Later', Misty's bike is repaired. Ash hides his sorrow by being happy for her, but Misty misunderstands, runs out and wonders aloud if none of the adventures they had meant nothing to him. there has been 'Misty's Song' (see http://downloads.khinsider.com/game-soundtracks/album/pokemon-2.b.a-master for free download), that implys her love for him, but it was never used in the anime. It has, however, been sung in 'Pokémon Live' by Heidi Weyhmueller, but the musical isn't considered canon to the anime.