A person whose main focus in life is self-improvement will feel the greatest joy on days when his tests are the most difficult.
A righteous person once said that the happiest moment in his life was when some people tried to humiliate him in an extreme way, and he was able to remain calm and serene. He realized on that day that he was able to cope with insults with total inner peace.
Difficult life situations are not a contradiction to having a pleasant life. Rather, the ultimate level to strive for is when all difficulties serve as tools for joy and happiness. This is an essential Torah concept, which guarantees that if you follow it 100 percent, then your life will be 100 percent full of happiness and joy.
(see Rabbi Yosef Hurwitz of Nevardok - Madraigos Haadam; Rabbi Pliskin's "Consulting the Wise")