what changes so deeply about a relationship after marriage?

im getting married in 9 days. i live in a pretty small town and work at a grocery store so pretty much everyone knows. im currently a 25 y/o female and my fiance is a 31 y/o male. people are constantly telling me were "in the honey moon stage, things are gonna get hard, you won't want to be around eachother anymore, your relationship will change" but i can't figure out why any of that has to happen unless you stop appreciating your partner. see ive lived with my fiance for 4 years. we've basically been living like we're married for nearly half a decade already. i already do his laundry, clean his house, share an account, and am on his benefits as a spouse. during the covid lock downs we were the only people eachother ever saw for 7 months straight and we both recall it to be one of the best times in our relationship. we still go out on dates, buy eachother gifts, do special things for eachother and love eachother as much as we did in the beginning. i don't get why people are so convinced that relationships have to turn sour because you signed a few papers. can someone please explain what changes so deeply after you simply make things legal?