He never used to believe in soulmates. She asked him once if he did and he'd shut her down with his shallow defense tactic. Because, truthfully? He was beginning to. And it scared him.
Not the 'one person you're fated to love' kind of soulmate. There was too much chance of missing the mark in that. For that, Kate had to die so they could meet.
Instead, he believes in the soulmate your heart commits itself to. It could have been another, then, but now his heart wears blinkers, his compass points to a single person.
He gets it.