The pain exists and happens.
When we experience pain our self-preservation generates fear to prevent that pain have to repeat.
When we are afraid, our survival instinct generates the strength to defend ourselves, we usually call this power anger, using it during conflicts.
If our power leads us to solve the problem, then we have been successful, which is satisfaction and happiness.
The self-preservation instinct makes us remember and to prefer the favorable situations. Thus instinct generates the desire that success is maintained.
The excessive desire leads to the concern, the lack of desire leads to depression, saturation of desire leads to boredom, these are all pain.
This is the cycle of nourishment of the five elements law.