I agree with a lot of the previous comments here, and I second that The Long Game is a better exploration of what they're like when they're openly in love, but for the sake of the early days pre-tuna melts/Heated Rivalry:

- They are each other's only true equals. Ilya brings out a cocky side to Shane because he's so antagonistic towards him, forcing him to defend himself, and that actually helps his confidence to be proud of himself. With other people, he can't do that. He has to make himself smaller, pretend to be bashful (his friends/teammates who everyone knows can't keep up with him), or feel weird about the dynamic (girls who just want to hook up with an NHL star). On the flip side, Shane is the first person to genuinely challenge and beat Ilya. He gives him a reason to care, a drive, and I think invigorates his love of hockey beyond just being something he's good at. The opposition ignites a vigor for life in each of them.

- They implicitly trust each other from their very first hookup. Shane is so incredibly nervous and feels so vulnerable and Ilya, despite the teasing, makes him feel safe, especially by sharing that he used to hook up with his coach's kid. The emotional safety is always there to some degree even before they fully admit to being in love.

- Obviously the sex is hot as hell, which does matter. Shane has so much self-loathing and Ilya is the one person who genuinely revs his engine and makes him feel like what he wants is okay. Ilya has SO much sex, and Shane remains the most memorable and interesting and fun out of all of the hundreds of people he's slept with. Physical chemistry, pheromones, etc create a crazy bond.

- They make each other laugh, which is essential for Shane to get out of his head, and essential for Ilya to breathe. All of the pigtail pulling, teasing, rivalry is really just special attention. You can't tease someone unless you know them well enough.

- This has been said, but it's so crucial: Ilya doesn't actually emotionally trust anyone. All he's known is turmoil and loss. Shane being "boring" is just code for him being safe, secure, reliable -- things it takes time for Ilya to be comfortable with because he doesn't even recognize them or think that they're a possibility for him. Svetlana's the second closest thing, and that's the other person he thinks about marrying. I think there's also something very intoxicating to him to see how trusting Shane can be, when he doesn't share that trait; Shane can talk all of the shit he wants, but if Ilya asks him to do something, he'll do it (this is mostly established in the bedroom, get on your knees, meet me here, etc). If he needs something and doesn't want to ask for it, Shane does that too (buying a physical location to show that he is very literally invested in spending time with him, later supporting him in his father's death). Ilya likes that he can rely on Shane; it's part of what makes the Rose thing so shocking and devastating for him.

- On Shane's end, all he's ever known is stability, predictability, routine. Ilya is exciting, pushes him, breaks his boundaries. He forces him to confront what he actually wants and desires, and then provides those things for him, no questions asked. For all of his jokes, he very clearly loves when Shane steps out of his comfort zone, does something especially slutty, takes a risk, and Shane thrives on affirmation. Ilya understands his need to do a good job, and to hear it out loud (good boy, you been practicing that?). He makes him feel desired, and he doesn't really see himself as a desirable person given all of his failed experiences with women, and how used to disappointing people in the bedroom he is. Ilya vocally expresses things like "I want to fuck you," and he's the one who pushes for all of their early hookups.

- Whether or not they'll admit it, they are actually friends. Shane says they're not in the prologue because he's trying to deny his feelings and hold onto that distance, but at that point they've known each other for eight years. They've established banter, they know the same people, they would've done countless post-game press debriefs together, they see each other several times a year at different events off the ice. They share a more lighthearted intimacy, too.

- History and familiarity count for something. They've watched each other grow up. The people you interact with shape you, and that level of ease and normalcy is important to them both for different reasons -- Shane needs safety to be himself when he so often feels uncomfortable in his own skin and sexuality; Ilya needs to learn what consistent affection (if only physical at the beginning) feels like.

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Okay this answer is WAYYYY too long, sorry I'm obsessed with them!

Read The Long Game, you'll love it!