Luke 19:1-10See how God loves what is small, who is humble, and how he blesses them.
Zacchaeus is in the same predicament. Aside from being small in statue, his dignity has fallen considering the ire of the Israelite for being a tax collector. But Jesus called him by his first name and even invited himself to his home. Zacchaeus was indeed grateful that he offered to pay four times the amount he stole and give half of his belongings to the poor.
These are the signs of a grateful heart:
Remorse - the sinful man realizes how much he offended God not only because he deserves just punishment but more so, he offended God who is all-good and deserving of all his love.
Amendment - the sinful man experiences how he is deeply loved by the Father despite his sinfulness that he is willing to make amends, correct all wrongs, and restore things and relationships to their proper state.
Offering - a grateful heart is an offering heart that loses all selfishness worsened by sin and releases the person from further slavery to it. He gives his life back to God and offers his services to others.
Gratefulness is the experience of heaven itself as nothing can ever separate us from God.