think of it as the concept of your childhood best friend but extended to the future. they're going to be different from your high school best friend because your childhood best friend not only knows of your past but lived in it. so reader is loosing out these moments that will shape (and the worse to happen is to drastically change) tighnari when he returns.

tighnari manages to allay their fears by reminding them that there will always be more years for them to have fun, and that the changes will not be something big in the long term, but when tighnari comes back he has a lover, and things between them DOES change. they have been proven right.

it's not really the "you're mine you can't have anyone else" typa beat. friends having a lover does change things in miniscule ways. they have more responsibilities, more things to be concerned off, and chances to hang out with them are few and far between.