You get good at exactly what you practice, and you are always practicing something, whether you are aware of it or not.
Some people, over the years of intense and labored practice, have become grand-master experts at feeling sorry for themselves. Of doubting themselves. Of self pity.
Practice anything enough and it becomes automatic, effortless, and instant. Whether that be joy, or anger.
Like a weightlifter’s back, which naturally falls into a posture of stability and optimal leverage when doing every day tasks, one is capable of falling into states of happiness and gratitue, but only after assiduous practice.