[Madeline] was the person who understood him! Here, where every one snatched at her and Dr. Brumfit beamed on her with almost matrimonial fondness, she was precious, she was something he must have. (5.2.7)
It's hard to tell whether Martin loves Madeline as a person or as a possession. It might even be both, but there's no denying that he's more attracted to her when other men are hitting on her.
"Darling! I almost feel as if I dared to love you. You're going to marry me and—Take me a couple more years to finish my medical course and couple in the hospital, then we'll be married." (5.5.15)
Notice here that this is the closest Martin ever comes to telling Madeline he loves her. But the thing is that he never does. He only says that he almost feels as if he dares to love her, which isn't the same thing. From the get go, you can sense that things aren't ideal for Martin and Madeline to be together forever.
The returning train was torturingly slow, and he dismounted at Mohalis fevered with visions of [Madeline]. (6.1.6)
Martin might love him some good science, but he also gets really lonely when he doesn't have any female companionship in his life. That's why he finds the train ride way too slow when he's on his way back to Madeline after summer vacation.
"Oh, I mean—I don't mean—But I do hope this Tozer person—I'm sure I shall like her, if you do, but—I am your own true love, aren't I, always!" (6.5.12)
When Martin asks Madeline to meet Leora Tozer, Madeline has no clue that she's about to meet Martin's other fiancé. But part of her knows that something isn't right, because she takes this opportunity to ask Martin whether she's his one true love.
"I wasn't jealous—Yes, I was. Oh, I can't help it! I love you so much. I'd be one fine lover, now wouldn't I, if I never got jealous!" (7.3.19)
When Martin sees Leora dancing with other men, he can't help but feel crazy jealous. But for him, this is just a sign of how much he loves Leora. If he didn't love her, he wouldn't care so much.
His head was buried in her lap and he clung to her affection, the one reality in a world of chattering ghosts. (19.2.32)
Martin always comes running back to Leora when things aren't going well in his life. Sure, he spends all of his time working and experimenting. But if something goes wrong, she's always there, waiting for him and ready to give him a shoulder (or lap) to cry on.
The seedy Pickerbaugh domain was enchanted […] and over all the world was the proper witchery of moonstruck love. (21.4.35)
Martin doesn't like his new boss, Dr. Pickerbaugh. But he does like the guy's teenage daughter, Orchid. It's not clear whether Lewis is being tongue-in-cheek when he says there's love between the two of them, but you can bet there's a whole lot of lust on Martin's side.
Because he loved her and also was fond of her, he was annoyed when she was less sleek, less suave, than the women he encountered at Angus Duer's. (25.2.2)
It's because Martin loves Leora so much that he hates when she shows up to dinner parties dressed in ratty clothes. For her part, Leora doesn't think that looks matter. But Martin gets mad because he wants everyone to find Leora as beautiful as he does.
He sat up, aghast with truth. Was he going to fall in love with this desirable and quite useless young woman? (34.7.31)
Martin is mortified when he feels like he's falling in love with Joyce Lanyon. Now don't forget that the two of them will eventually get married. But at this point, Martin finds her desirable and useless, which is really harsh. Basically, the book is saying that Joyce is a beautiful woman with a lame personality.
He talked to her, his voice a little insane, trying to make her understand that he had loved her, and had left her here only for her safety—(35.5.9)
Martin is beside himself when he finds his wife Leora lying dead in her bedroom. He has been away trying to cure people of plague. But while he's been away, Leora has been dying and wondering whether he'll come back to save her. It's fair to say that Martin is done with love once Leora is gone. The only thing left for him is science, even though he does get married again.