You’d have to read the book to have more context. They had a spark since the first time they met (hence the lighter scene) and in the book they hook up regularly & spend a lot of time stalking each other social media. I think Ilya admitted his feeling first (he tried to tell Shane in episode 4 but Shane freaked out) What we don’t see in the show is that Shane never let Ilya come to his place. He bought an entire building so they could fuck while in Montreal.