Mc might love her but he isnt "in love" with cortana more like the love you feel for a squadmate or a best friend.
More or less, this.
Also, she's the one constant he's had in his life that was there with and for him. And that's throughout all the death and destruction going on around him all the time. Of course he'd love her. Who wouldn't?
I'd describe the feeling every Spartan has towards Dr. Halsey as love for the only mother they've grown to love. John has love towards Cortana as the love one would have ones sister. She is one of the few consistent things he's had his entire life. She's the one thing that he never expected to lose. Even his fellow Spartans came with the stigma that he may or may not ever see them again but Cortana is different. She seemed almost untouchable.
"****'em if they can't take a joke" ~John Travolta~ -------------------362010-------------------