Tom Hagen - I agree - maybe to Michael, love was a weakness. Well, maybe not a weakness but love was dangerous. IN GFIII, he reminded Vincent, they kill the ones you love and yes, as he was in love and happy with Appolonia, bam! they kill her.
But love and mistresses are not the same thing - I think MIchael was too stressed out and tired for a mistress- but maybe if he had the excitement of a mistress, he would have been a lot happier!