Luke has been noshing on some serious sour grapes. He got dumped and decided that "locking himself off from the world" (45.4) in a creepy, furniture-less tower wasn't enough: he had to start hating pretty girls for no reason, too. For most of the novel, his attitude toward Lucy is the worst thing about her awesome job. In other words, he takes forever to get over being dumped and is not a nice guy while he's doing it.
Luke's transformation is a little weird, but bear with us: one day, he wishes for a child. Guess God was listening, because this crazy girl Skype shows up. He starts gradually getting nice, and then he has a revelation:
He would like to wake up with someone, a woman worth crawling out of the warmth for. On a morning such as this, he would pad through to the kitchen to put the coffee on, present it to her as an offering. He would happily suffer the cold floor beneath his feet in exchange for the happiness of returning to bed for a few minutes to drink coffee and talk. (45.4)
He wants a new girlfriend. To be honest, we have no idea what causes this change. Is it the healing power of time? Is it Mr. B? Dunno. But whatever the cause, he starts acting super nice to Lucy and, by the end of the novel, they're skipping off to work hand-in-hand.