Misty is an image that Ash had of a girl. This is why she plays so prevalently in the series but is ultimately unattainable before the coma. Likely the one that helped get him to a hospital. I have a theory in line with this: Since Misty was his initial love interest (if only subconsciously), he needed her to reach a level of womanhood. He felt that people could only have relationships when they've matured. But in practice, it turned out he couldn't cope with it and just wanted the normal, pushy, arrogant Misty he knew, and wouldn't let her keep Togepi anymore. Misty is Ash's first attempt at a girl he could love, however, being a girl from the real world, that all he knew of her was her anger, she ended up very angry. Constantly berating his sexuality, but eventually mellowing out until she had faded into the background. This was also traumatizing to him, being attached to it. Since then, the thought of anyone around him maturing to adulthood has been blocked, and anyone who shows signs of it will quickly end up leaving for another, more naive fill-in.
Max came with May, she played the ID with great aspirations, and he played the sensible Ego that "Session". They worked for a little while but Ash, being a teenager eventually his sexuality had to come back into play. He kept reinventing himself and eventually wrote new aspects, but his mind slowly brought back the old ones as a crutch to make the transition easier.
Dawn is Ash giving himself a chance to love. Since he already established Misty as someone he's not likely to go anywhere with, he created a new super female, one that was more like him, and less violent all the time. (One will note that both May and Misty had no tolerance for Brock whatsoever whereas Dawn seems to try and shrug it off.) Richie and his Pikachu were another success story for Ash, but he wanted one he could be closer with. One nearly identical to him. One that even used a similar roster to him.