The causeless mercy of Kṛṣṇa, the dust of His lotus feet, His prasādam and association with His devotees are some impetuses toward a devotee's engagement in transcendental loving service to the Lord.
Kṛṣṇa exhibited His causeless mercy when He was present at the departure of Grandfather Bhīṣma. During the Battle of Kurukṣetra, Bhīṣmadeva, the grandfather of Arjuna, was lying on a bed of arrows before departing from this mortal world. When Lord Kṛṣṇa, Mahārāj Yudhiṣṭhira and the other Pāṇḍavas approached Bhīṣmadeva, he was very grateful to Lord Kṛṣṇa, and he addressed the brāhmaṇa military commander, Kṛpācārya, thus: "My dear Kṛpācārya, just see the wonderful causeless mercy of Lord Kṛṣṇa! I am most unfortunate. I have no qualifications. I was opposing Kṛṣṇa's most intimate friend, Arjuna—I even tried to kill him! I have so many disqualifications, and yet the Lord is still so kind that He has come to see me at the last point of my life. He is worshipable by all great sages, but still He is so merciful that He has come to see an abominable person like me."
Sometimes the vibration of Lord Kṛṣṇa's flute, His bugling, His smiling, His footmarks on the ground, the transcendental fragrance of His body and the appearance of a new cloud in the sky also become impetuses for ecstatic love of Him.