One must learn to smile.
It is not advisable to do everything grimly and fearfully.
Some things can be taken more lightly, and in many contexts cheerfulness is permissible.
It is not only a matter of avoiding sadness, which the Sages considered the worst of sins, but of keeping a sense of proportion, saving one's seriousness and grim determination for situations where they are needed.
Judaism presents a particularly difficult spiritual challenge, for it asks us to live a life of holiness, not in monastic seclusion, but out in the world.
It is a challenge that calls for balance and a sense of humor.
–Rabbi Adin Steinsaltz
From "Seriousness and Restraint" in Teshuvah by Rabbi Adin Steinsaltz