What is the Composition of the Heart?

But the Lord said to Samuel, “Do not look at his appearance or at the height of his stature, because I have rejected him. For the Lord sees not as man sees; for man looks at the outward appearance, but the Lord looks at the heart.”
1 SAMUEL 16:7 AMP

The heart is where motives are made. What elements go into that making? Hurt, offence, bitterness, rejection, grace, mercy, humility, self-control?

The more impurities left in a product the weaker it is. The purer it is, the stronger it will be.

What impurities will we allow to remain? Where is the line?