Ok I'm gonna ramble a bit because this is something I've thought about a lot and it's about my perspective of Soul and Maka's relationship
I think the cast of Soul Eater is young enough that Maka and Soul don't see each other as anything deeper than friends (or maybe they're completely comfortable with the friendship they have), but I like to think that at their age this is the border between childhood and adolescence. They're still "kids" so they aren't thinking of romantic relationships yet and still treasure their platonic friendships.
Also they're young, 14 or so, and they're still maturing, still kids, still wanting to be childish and immature and have their fun but at the same time they want to seem responsible and older and respected by adults. It's a pivotal part of puberty.
If so, I want to think that they start experiencing romantic feelings later on, at 19 or so. Maka and Soul felt something akin to attraction for the other as they grew but either they brushed it off or believed it was a combination of their close friendship plus partnership, resonance, etc. At 14, they were busy with their assignments and missions collecting kishin souls/becoming a death scythe. They didn't have time for that and never got around to it. With her job as a meister who hunted down people as a duty, it was all they ever thought about.
So when they grew up and graduated, became a death scythe, saved the world and maybe finally settled down, they had much more free time for other things. Maybe they finally addressed their feelings and entered a relationship.
Then again, I also like the idea that they were comfortable with what they had, i.e the idea that their friendship was more valuable and irreplaceable than an actual romantic relationship.
Or maybe Maka knew that she wasn't ready for a relationship with Soul and I think Soul respected that. It's just that they were basically in a war with Asura and Medusa and being in a relationship wasn't suitable at the time.
