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

Understand! What the innermost heart brings forth, one must ponder!

Through saving others (performing the Joyous Service), your body is saved (no suffering delivered by the soul/heart). 3-47

Interpretation:

The heart by itself brings upon our fate, caused by what we ourselves have done in the past (life). We reap what we have sowed.