Why jeans? you can wear jeans any other time on the ship. Why on formal night?. Don't you want to wear a beautiful dress or gown and show it off? Formal night is just that formal night. Trust me,it won't kill you to wear a nice dress, make the effort. I have dresses that I bought for weddings that I wish I had more formal events to wear them. Celebrity is more up-ascale so their dress code is in place for a reason. For me its a pride thing, I take the time and effort to get myself ready for formal night. Besides I love wearing the dress/gowns I bought, never get a chance otherwise.
I don't care how comfortable, your jeans, tank top or flip flops are; they are not for formal night on any cruiseline. It screams I don't want to bother to make an effort to dress up. Its 1 or 2 nights for a couple hours you can survive the world won't end being dolled up for a couple hours right? just my opinion.