You know, I recently realised something whilst half-asleep.
May, Dawn, Iris and Serena owe Misty a lot.
When we first met Ash, he was arrogant, rude, ignorant about feelings, and took every chance to insult Misty.
Example: In 'The Battling Eevee Brothers', Ash told Misty she could pass for his brother. Which is a horrible thing to say to a young girl.
He was even rather violent, and tried to solve all of his problems with either his Pokemon or his fists.
Example: In Electric Shock Showdown, he even tried to punch her!
However, by the end of Johto, he was much calmer, thoughtful, had started complimenting Misty, and had even started realising that he wasn't hot shit, he was just a kid who still needed a lot of work. He started thinking more in battles and using strategy. He also toned down his violent tendencies, and stopped trying to punch his way through any challenge when seperated from his Pokemon.
By the time he had started travelling with May, they only really argued a couple of times. One of those was because of a misunderstanding about how double-battles were supposed to work. (And considering that was their first attempt at it, it's understandable they'd mess it up.) I think the other was because she took too long to get ready one morning when he wanted to get moving to a gym battle.
Which is understandable, because he was used to Misty, who probably was ready quickly in the morning after a quick brush of her hair and tying it back.
Ash's main male role models were Gary, and establish arrogant jerkass, and Brock, a complete perv.
If Ash had never travelled with Misty, he possibly would never have learned to respect people's feelings, and would have learned Brock's lame pick-up lines. He would never had learned that other people's feelings matter, and that pick-up lines never work. (Well, Brock's lines never work...)
Ash would still be an arrogant little bastard with no respect for others, and he would have caused May, Dawn and Serena to run away from him n tears within a week. Iris would have pounded him into the ground, and then stormed away muttering about stupid little kids.