Happiness of The Spirit Soul
The spirit soul can be happy only in the association of the supreme living being, the Personality of Godhead, and nowhere else.
Srimad Bhagavatam 3.2.11,
Srila Prabhupada.
The more we are happy in Krishna Consciousness the more we become purified.
Without the grace of the Lord, no one can become happy and prosperous.
Srimad Bhagavatam 3.1.26,
Srila Prabhupada
That procedure which in the beginning may be just like poison but at the end is just like nectar and which awakens one to self-realization is said to be happiness in the mode of goodness.
The happiness in serving is the happiness that material nature cannot impede because we can always serve in some way or the other.