Joy is a fruit of the spirit and also a basic mark of christian.
Natural tendency when one is under stress is to be unhappy and troubled; but if we obey the commandment of "Rejoice always" just like Paul and Silas did in the prison they are in a very distressed situation but they prayed and give praises to God this led to chains broken, locked doors open and joy overflow.
it makes our faith very strong .
When I am unhappy, distressed and after some time I fall back on praising God and all things will be come to normal and the burden becomes lighten.