It is the heart (soul) we must understand to find out why we suffer or have joy in our lives. The following is from the Ofudesaki, the most sacred scripture in the world
This sweeping thoroughly accomplished, must be done.
The truth of the heart, because there is no understanding. 5-28
Understand! What the innermost heart brings forth, one must ponder!
No one knows the heart. By understanding our own heart (soul, spirit), we can sweep our minds of evil thoughts. These thoughts are regret, covetousness, greed, arrogance and self-love. These thoughts put into action causes suffering in the world. But unknown to most people, these evil actions cause suffering back to oneself.