Benyamin Eckstein posts to HirHurim: The big fear which prevents people from studying and applying Mussar is that they are deathly afraid of getting to know themselves. It is that which is too painful, not hearing about the dire consequences of sin.
The joy and inspiration are usually a kosher way to avoid and blur the pain of deep, methodical, honest self-awareness, appraisal, and improvement.
ChareidiLite posts: Haven’t there always been different styles of Mussar? Rav Yisrael Salanter was always trying new approaches.
The Narvadok strain was more the fire and brimstone style (how insignificant and failure prone is man), while the Slobodka style was more positive (look at what great potential man has).
In The Making of a Gadol, Rav Kaminetsky implies that the Alter of Slobodka moved away from the Narvadok mussar style after his son went off the derech and became a communist, realizing that a different approach was needed (at least for some).