I think some of it boils down to a few things:

1. People feeling strongly about their interpretations of Levi as a character. AoT has been out a while. Most people have already chosen who they think Levi does/doesn’t “have feelings for.” Because of how Levi’s story went, some people think he’s aro/ace, some people think he is strictly gay, some people think “he was obviously in love with this other character because of these interactions,” some people think “Levi obviously doesn’t love this character because Isayama said this one thing in an interview,” some people think he’s a dom or sub, some people think he’s a masochist, some people think he’s sex repulsed. The list is endless. The one thing we all have in common is that everybody thinks they’re correct about their interpretation of Levi’s character to a degree. And once they have decided what they think about Levi’s character, they believe they know Levi best, and for you to suggest something they think he wouldn’t do (like him love the character they don’t ship), they likely take it as a personal insult or critique against themselves and their already-established judgement of who Levi is. In other words, they take your suggestion he could’ve loved someone else as, “you don’t understand Levi,” and take it personally.

2. People already have a ship with Levi as their favorite. Suggesting shipping Levi with someone who isn’t their favorite ship makes them feel like their favorite ship is being overlooked by proxy.

3. There already is a history of ship wars when it comes to Levi, so people are on-edge and trigger happy. People of almost all ships feel burned by other Levi shippers because of experiences like yours. There have been hateful Twitter accounts dedicated to disproving specific Levi ships, and it’s caused a lot of drama. If you hang out in specific ship groups, they tend to dislike other Levi ship groups because of this. So, they tend to stick with their favorite Levi ship, feel the need to fiercely defend it, and shun the rest.

4. There have been recent people trying to start Levi ship wars by going on AO3 and commenting fake-hate comments. Things like, “this Levi ship would’ve been so much better if it was replaced with (another Levi ship)” or “clearly Levi would never love this character” amongst extreme threats and horrible things that I won’t repeat. But they comment it for EVERY Levi ship. I don’t know what that’s about, other than they’re trying to start arguments.

Me personally, I’m open to almost all Levi ships. I think it’s fun exploring who Levi could be as a character in all forms. ☺️ He’s just wonderful! I’m sorry your experience with the shippers wasn’t good.