As someone who’s ace/ND/queer, for me, it is often a matter of “Do I simply like to look at this person?”, “Do I want to be this person or otherwise emulate an or more aspect(s) of them?” and/or “Do I want to be with this person?”
Also, I suppose, having a traumatic past can make love a really difficult concept to grasp. Take for example this quote from Benjamin Alire Sáenz: “But love was always something heavy for me. Something I had to carry.” In some way, this is true for me. I instinctively dread other people having crushes on me or expressing any kind of attraction because I always get the impression that I am obligated to react in a certain way which, of course, is absolutely foolish. But when the love you’ve been given is conditional or when you have to “work for it” from a young age, that distorts your view of it. While I absolutely empathize with the take that “Love is a helium-based emotion” from an intellectual standpoint, in real life, love still feels more often heavy to me than not.
Finding their appearance or little details about their personality pleasing. Personally, I wouldn’t settle for obvious clichés here like rock-hard abs, a firm ass, or big breasts. Nuance matters. Familiarity, growing or already existant. Liking that one crooked tooth that only shows when they are genuinely smiling, for instance. Show us that the MC pays attention to this person, is maybe seeing them more complimentary than others do. That they make an effort for them and maybe, specifically, why they make that effort. (The answer doesn’t have to be: “Because I’m in love with you.” The answer could be everything ranging from “You inspire me to be better” to “You’re the only person I know who can belch the alphabet backwards, and it seriously makes my day every damn time”.)
For me, as a reader, it would be important to feel a certain “vibe” between these characters. Do they work together on an interpersonal level? Are they nourishing each other intellectually? Are they equal in this relationship? Can they be tender and vulnerable with each other? Can they butt heads, challenge each other and still never lose sight of the mutual respect that binds them? Show how this other person energizes the MC. But also what work both willingly put into the relationship that might strike other people as “not typically platonic”. While the MC may be unaware of their attraction, never forget to also elaborate on the things the MC is instead aware of and deliberate about in this relationship.