I think a person should be happy all the time because one should keep in mind that life is not under fully controlled by a person. So one can't do anything. So if happiness is temporary then hard times are also temporary so one should be satisfied with this belief that one day will come for rejoice.
I never lose heart because I am thankful to God that atleast I have things which people mostly don't have. At the moment, I am jobless but I am still happy and thankful to God and high hopes that my good days will come soon. If I don't belief this and think that everything is under my control then I may have killed myself.
So always be happy and even be helpful to others.