The series is about the people around her changing Frieren herself and making her value the smaller things in life more, and it uses loss and melancholy as powerful devices to do so, with her going on this journey of (self)discovery because because she realised that humans are short lived and yet she never tried to learn more about her friends that she defeated the big bad evil guy with who she spent a decade or so with.

It's different to To Your Eternity which is also partially about its nigh immortal protagonist's - Inno's - growth as a person, but while frieren is growing from someone who just goes through the motions to someone who values human experiences, Inno is a blank slate learning to adult.