Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 36

Sanskrit:

तंसुखाराध्यमृजुभिरनन्यशरणैर्नृभि: ।कृतज्ञ: को न सेवेतदुराराध्यमसाधुभि: ॥ ३६ ॥

ITRANS:

taṁ sukhārādhyam ṛjubhirananya-śaraṇair nṛbhiḥkṛtajñaḥ ko na sevetadurārādhyam asādhubhiḥ

Translation:

What grateful soul is there who would not render his loving service to such a great master as the Personality of Godhead? The Lord can be easily pleased by spotless devotees who resort exclusively to Him for protection, though the unrighteous man finds it difficult to propitiate Him.

Purport:

Every living entity, especially persons in the human race, must feel grateful for the benedictions offered by the grace of the Supreme Lord. Therefore anyone with a simple heart of gratefulness must be Kṛṣṇa conscious and offer devotional service to the Lord. Those who are actually thieves and rogues do not recognize or acknowledge the benedictions offered to them by the Supreme Lord, and they cannot render Him devotional service. Ungrateful persons are those who do not understand how much benefit they are deriving by the arrangement of the Lord. They enjoy the sunshine and moonshine, and they get water free of charge, yet they do not feel grateful, but simply go on enjoying these gifts of the Lord. Therefore, they must be called thieves and rogues.