I think that pre-Sollozzo Michael loved Kay as a rebellion against everything the Corleone Family stood for. She was an escape hatch, a path to a different life.

After he returns from Sicily with his big five year plan to legitimacy, I think he loved everything that Kay stood for: that WASP lineage that no money can buy. She brought something to the table that he could never achieve. She was educated, beautiful, American (in the DAR way), and she would be an asset by his side in his new legitimate role.