My view:
Michael certainly loved Kay before he got involved in the family business. We see it in a deleted scene from GF in which he and Kay are cavorting, playfully, in a NYC hotel room just before he learns Vito has been shot. But, his experiences after that hardened him and stripped him of sentimentality. His "courtship" of Kay in New Hampshire was more of a business negotiation (exemplified by Michael's looking like a pint-sized banker with that ridiculous Homburg hat, and being followed by a limo). His "love you/need you" was emotionless. It was pretty obvious that he saw Kay as an important to him only or mostly as an important stepping stone to "legitimacy," not as the object of his love and passion--much less anything like the "thunderbolt" that hit him when he first saw Apollonia. He continued to use and manipulate Kay throughout the rest of GF and II, ultimately slamming the door in her face, as Talia Shire reminded us in another thread.

And yet...

...by III, he's back to courting Kay again. Why? Did he see reconciling with Kay as the last step in his getting out of Mafia life and becoming truly legitimate? Was he looking to manipulate Kay yet again as a late-life personal triumph? Or, did he find his early love of Kay somehow rekindled? I'm inclined to agree with Pete and Oli that Michael was too narcissistic, suspicious and manipulative to truly love any woman, but I'm not dismissing the possibility that Kay was an exception.