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 filter, where do you think it is.
The heart and the mouth (actions) are the sand and filter. Ofudesaki 3-11
God is telling us that the heart is like the sand in the waste removal system. It collects waste. What could this waste be? Could it be the demerits of our evil actions? It collects and stores it. What does the heart do with these demerits? More to come!