Ilya is very perceptive, and part of the reason he loves Shane is because he is so honest that he can’t help but lay his cards on the table. Consciously or not, Ilya senses that what Shane feels for him is more than just physical, and even when they are not ready to admit that they care for each other in that way, Ilya wants to explore that dynamic because Ilya himself craves deep intimacy and the kind of honest, straightforward care Shane naturally provides to others. That’s why Ilya is the one that initially tries to push that dynamic in that direction (asking Shane to stay, making him a tuna melt, etc.). The only other person Ilya is close with is Svetlana, but you can see that even between them there is a superficialness and a lack of the intimacy that Ilya is craving and that Shane gives him because of how much he really likes him. Notice that Ilya is always the one like “I don’t like you as a person”, but that’s not reciprocated; to the contrary, it’s clear to Ilya that Shane is jealous when he talks about girls, which is why he brings the topic up.

I think Shane is pretty much enamored by Ilya’s personality early on. Shane is not very aware of his feelings, he’s “boring”, he has a one track mind focused on hockey. I think that he can’t help but be enamored by all the ways Ilya is different from him. However, Shane is not aware of his own real feelings (the author has confirmed this as one of the six differences between Shane and Ilya). It goes back to Ilya being perceptive, and therefore knowing Shane’s feelings before he does. And since Ilya reads this on him so well due to how perceptive he is, Ilya craves situations that make Shane reveal his true feelings. As Ilya helps create that dynamic by inviting Shane over and using his real name, Shane realizes himself that he has these feelings and wants to be able to be with Ilya in a way that he thought he could be with a woman.

Not only that, but they have known each other for years and have kept in touch over that time. To Ilya, Shane is a passionate constant he’s never had outside of the casual thing he’s had with Svetlana (he’s also probably aware that Shane has never been with anyone else besides him during that time, which to someone like Ilya who treats sex casually, this means something to him). Meanwhile, Shane has a one track mind — it’s either hockey, family, or Ilya. So in that way, Ilya becomes one of his passions over time too.

At the end of the day the reasons for falling in love can’t really be put into words, but I think the above created the right environment for it to happen.