Grateful Heart
A grateful heart is not the one that is very choosy about what to be grateful for.
Supersoul
Lord Krishna, the Supersoul, Knows everything in everyone’s heart. He Knows everyone’s determination and everyone’s want.
Krishna, The Supreme Personality of Godhead, Chapter 81,
Srila Prabhupada.
Special Mercy
To show them special mercy, I, dwelling in their hearts, destroy with the shining lamp of knowledge the darkness born of ignorance.
Bhagavad Gita 10.11,
Srila Prabhupada.
Making The Heart Soft
One might become hardhearted by strictly following the rules and regulations if one is too eager to follow rules and regulations just for the sake of following. If one follows the rules and regulations strictly to please Krishna that makes the heart soft.
Lecture on "Great devotees exhibited ecstasy but we shouldn't" – Salem – Nov'06
Ungrateful And Grateful Heart
Ungrateful heart means miserable frustrated life and grateful heart is always happy.
Grateful Heart
Grateful heart is the heart in which seed of bhakti will grow.