Several reasons Frank.

At first, when Michael is in Sicily and meets Appolonia, there is no question that he was indeed struck by "the thunderbolt." He instantly fell in love with her. Combine that love with the fact that during Michael's stay in Italy, he began to learn about his culture and his roots. He returned to his family origins. During this time Michael knew that he had now crossed over and that when he returned to America he would be in the family business. And when he did return why not do so with a woman that he loved who was much like his mother. A woman that was from the old country, one who would be the dedicated wife and would never interfere with the business that Michael would return to and be a part of. Appolonia was the perfect wife for the boss of a mob family...and Michael truly loved her. But as we saw, things didn't go as planned.

Now Appolonia was gone and Michael returned to America without that perfect old world woman that was perfect for the new life that he would lead.

So now Michael is being schooled by his father. Only thing is that times had changed since Michael left to hide in Sicily. Things were a bit different. And while Vito was schooling Michael to become the new Don, the new Godfather, the Boss of bosses, deep down inside it was not really what he wanted for Michael. Both Vito and Michael realized that once the family regained the power that they once had, they would also have to try and legitimize the family in many ways. It was something that Vito had always dreamed about. And it would now be something that Michael fooled himself into thinking that he could actually become....a legitimate business man.

So he and his father devise this plan and after about a year or so of planning, Michael goes and seeks out his first love, Kay. I believe that Michael really loved her at one time. Being that Michael now deided that he would attempt to become a legitimate business man once he regained the Corleone power, he probably figured that Kay would be the perfect wife for an 'Americanized Legitimate Businessman'. If Michael had to entertain Seantors, Congressman, Corporate moguls, etc. then having an Americanized Wasp educated woman at his side could only help his image. Having a Kay Adams type woman at his side would definitely help neutralize the Italian gangster hood image that his kind of people resonated. At least in Michael's mind.

This is why I believe that Michael HAD to go back to a woman like Kay after being with a woman like Appolonia.

Remember, it was not like he ran to Kay as soon as he returned from Italy. He waited a year or more. He only reached out to her after realizing that his first love would be the perfect American wife that he would now need to help legitimize himself and his family.