I hate answering this stuff because it inevitably sounds like hate but I keep getting asked this so maybe this will stop those questions lmao.
disclaimer: I dont intend this as hate because everyone should ship as they please.
but as to why I dont personally ship it; I consider eren x levi to be a crack ship because it will never happen in canon, and there’s just no way I see a teenager and a 30+ year old man having a healthy and functional romantic or sexual relationship, particularly when they have really no significant relationship with each other outside of captain + subordinate or mentor + student. that’s not to say there’s no meaningful or important feelings between eren and levi, and when levi tells eren he’s sorry he’s always the one who has to fight in the recent chapters, I got a little teary tbh - but generally I see their relationship restricted to their working life, there’s not really a significant or typical friendship there, particularly not one that would provide a foundation for romance.
the whole gross age difference and waxing poetic about how age doesn’t really matter when its “love” aside; maturity and life experience DO matter, and you find that out as you have more relationships and more dealings with romance and dating.
maturity, age, and life experience really do inform who you are and what you value and how you behave and almost every other aspect of your life - and levi and eren are too far apart in all those aspects to connect on a meaningful romantic level. especially considering levi; he’s been through so much, more than most experience in a life time (so has eren but he still doesn’t come anywhere close to levi) and I think levi in particular needs someone with the wisdom and experience to understand him, to be patient, considerate, to want the same as him and perceive things in a similar light - to meet him on equal footing in most respects because levi would come with so much emotional baggage. he needs an equal; he needs someone even wiser or even more understanding than him in some respects because he likely has a lot of internal struggles, conflicts, confusion and difficulties navigating the more social and romantic aspects of life. even more well-adjusted people can struggle with romance, let alone someone like levi - and a teenager would not be able to provide him with that specific level of stability and understanding.
so this is one of the reasons I do ship him and hange; because they are equals (and because hange is such an empathetic and considerate person and canonically understands levi in a way nobody else does) and it’s why I think erwin x levi is a good and valid pairing too because erwin is also a more emotional equal to him.
but I cant ship eren x levi - or him with any of the 104th; I just think there’s absolutely no basis for it other than the fan art is probably hot or cute for a lot of people, which in itself can be really difficult for other people to accept due to the minor x adult aspect (I feel that way too tbh). but you know I ship levi x hange because it makes me super fucking happy and im sure its the same for a lot of peeps who ship eren x levi so I dont have a problem with people who want to ship it. I just cant get into it.