Hmmm... I'm just thinking, I myself hadn't had a partner yet, but some crushes, usually on friends as I don't feel very connected to lots of people that I could develope more "serious crushes" on them - and it honest to god is so different with every human being.
For one, physical attractiveness is kind of important, but also only so far. The first crush I had was because a) he was one of the few in my class I found physically interesting, unlike other boys which were rather bland imo, and b) because we were friends so we got along and didn't hate our guts, and c) we actually could tease each other on an equal level which kind of equals 6th-grader flirting. He drove me mad because he had a lot of quirks and the amount of second-hand embarrassment was horribly high, but somehow I still liked him and admired him as he was as strong-willed as I.
Mutual respect for the other is really important for me, I noticed it a few years later with another guy who was such a pushover that I quickly stopped flirting with him. We were somewhat friends as we shared a lot of classes and worked on projects etc together, but as nice as he was, I couldn't respect him as much as I wished I could. I got along with his best friend way better, but there the physical attrection just lacked, even though we were good friends.
I also have some kind of a crush but not really on my very best friend, with her it's more like a mind-play that started it for me, I guess?
Like I wouldn't mind to date her if she'd ask me to, but it's also more than okay if we don't since I love her deeply either way and sherish the friendship that we have. She is propably the closest to a soulmate that I know, so it's running really, really deep and we went through a lot. I guess I don't care anymore what form this love would take? I could see myself raising kids with her, or she is my best maid at my wedding, I can't tell what I'd want more. I mean, we treat each other almost as though we were already married, just that it's currently entirely platonic. It just hurts to be apart, and when we're together, we can do the most mundane things as sitting on the sofa and watching tv but it's still kinda great because it's us.
This feeling of mutual likeness is so hard to describe >w<
AND: the less clear I can imagine the relationship with someone else, the more authentic my crush/love actually happens to be! Because there are so many possibilities how we could end up, what we could do, how we could develope, so many things to still be discovered, that it's hard to predict anything! The faster I know what turn this thing takes, the more bored or even panicky I get, especially if I feel it is going into a direction where I personally never wanted to end up, so I get out real quick, even if it's just a potential relationship.
As for chemistry, I think it's how people actually work together, and if there will be a dynamic relationship even after years and years of knowing each other just because of how their characters are, or if, as soon as the novelty of a relationship wears off, they kind of just kinda live but don't discover anything new. I think that's why the "best friends" trope is such a popular topic for romantic relationships? The friendship is some kind of a constant, but the characters still grow and grow together in a very unique way.
To determine whether the romantic/sexual relationship is authentic, it's kind of wiser to think from the characters' pov and what motivates them to get into a relationship, or if they are interested in any of these relationships at all. I don't like to put off something like romance because it's such a trope and we've seen it a thousand times, but rather because in that situation, the character's thought definitely wouldn't be about romance. In that case, really, think of the character first, and then what the audience wants or doesn't want.