Material sense gratification, sinful activity kills the propensity to serve Krishna.
Every living being is anxious for full freedom because that is his transcendental nature. And this freedom is obtained only through the transcendental service of the Lord.
Srimad Bhagavatam 1.6.37,
To use mind in thinking of Krishna is as equivalent as to engage body in the service of Krishna.
The Lord can be served simply by adoring Him and accepting Him.
Srimad Bhagavatam 2.9.36,
Prabhupada is most grateful when we render service to Krishna sincerely and it is by His mercy that we will be able to go back to the spiritual sky.