Jyn lived under the oppression of the Empire, her parents stolen from her and having to live constantly on the run for fear of being found. She can’t have friends or any real relationships, really. She has been in and out of Imperial detention centers most of her young life.

What does she have to be happy about?

Rey, while poor and lonely, lives free of worrying about a galactic oppressor (until BB8) shows up. She has a home and can be whoever she wants to be.

Criticizing Jyn’s character for being dour about everything seems to ignore what a true hellscape her life has been in the shadow of the Empire. She is a prime example of the hopelessness that Luke’s New Hope is intended to correct.