Ungrateful heart means miserable frustrated life and grateful heart is always happy.
Prahlad Maharaj made things very simple, if you want to be happy in this world, then simply stop trying to be happy in this world; then you would be happy, but as long as you are trying to be happy in this world, you have to suffer because it a place of death.
If Krishna is pleased, you will feel pleased. This is automatic. If you are not feeling happy, that means Krishna is not happy by your service.
“Sa vai pumsam paro dharmo”, Unless we get pleasure, we can’t do it
The Happiness that we get through material things is very transient. Therefore, intelligent persons seek for permanent happiness.
Real happiness is in service. Accumulation does not satisfy the heart.