As the materialists are thinking that happiness is all in sense gratification, they don't see the happiness in serving Krishna, that's where the real happiness is achieved in much higher relationship and higher purpose. Their thought is, "Alright Dharma, ardha, kama, moksha" now we will make money, enjoy our sense gratification and at the end of life go for moksha. Howvere in devotional service, we know about prema, the prema is actually the love for Krishna. It is the highest perfection and Krishna is the highest happiness also.
Our happiness, our joy is simply to know that we are doing the will of Guru and Krishna, that's all. We don't need anything else.
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