Simcha (happiness) is a key ingredient in personality. All other personality traits are less important than happiness. For example, if a person tries to be kind but does not infuse happiness into his acts of kindness, his efforts to do kindness will fail to some degree. If he does the kind act with a smile and an inner happiness, his efforts will have a greater impact. All of a person's traits, trustworthiness, wisdom, strictness, etc. are suspect if the person is not happy. The Chassidic masters say that depression, although not a sin in itself, is the greatest sin of all because it leads to other sins.