Thanks for your very thoughtful comment, Shyam! You've left some good food for thought, so I'll do my best to explain:
I did/do think of Alex as more than just an average friend. However, at the same time, there were/are other dimensions besides our communication, care, and respect for each other that make us feel less than 100% hearts-and-flowers romantic. As an example: for Alex, my unorthodox spiritual beliefs make me a little weird, but for me — since spirituality's a big part of my personality, worldview, and goals — it's very important to have a partner who can really engage with me on those things instead of somebody who just "tolerates" my airy-fairy thoughts and for whom I have to water down that side of myself.
I was also aware, from before meeting Alex, that I was capable of an even deeper type of romantic sentiment, and if I'm not feeling that depth of sentiment with a given person, then I feel like something is "missing" from the connection. For him, there's probably a similar feeling; we've both been in love in the past with other people. But for as much as we love each other, that more decidedly romantic, love-of-my-life feeling didn't take hold between him and me.
It's not that I found a better partner anywhere; Alex is a fantastic person and partner. He's not "lesser" in any way. It's just that, I suppose, good communication and respect are invaluable (and, sadly, a little rare) — but they should really be the baseline *minimum* for a connection. For sure, a relationship doesn't stand much of a chance if those things are missing; they are THAT crucial. But by themselves, they don't necessarily trump every other valid relational need.
Thank you again for your thoughts and your supportive words!