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.
In the Vidagdha-mādhava there is the following statement: "When Kṛṣṇa was playing on His flute, Baladeva very anxiously declared, 'Just see how, after hearing the transcendental sound of Kṛṣṇa's flute, Indra the King of heaven is crying in his heavenly kingdom! And from his teardrops falling on the ground, Vṛndāvana appears to have become a celestial residence for the demigods.'"
Ecstatic love for Kṛṣṇa, which is known as anubhāva, is symptomized by the following signs: one becomes engaged exclusively in the service of the Lord, being attentive to carry out the orders of the Lord faithfully; one becomes undisturbed and non-envious in full transcendental loving service to the Lord; and one makes friendship with the devotees of the Lord who are situated in faithful service to Him. All of these symptoms are called anubhāva, ecstatic love.
The first symptom of anubhāva, or engagement in a particular type of service, is exemplified by Dāruka, a servant of Kṛṣṇa who used to fan Kṛṣṇa with a cāmara, a bunch of hair. When he was engaged in such service, he was filled with ecstatic love, and the symptoms of ecstatic love became manifest in his body. But Dāruka was so serious about his service that he checked all of these manifestations of ecstatic love and considered them hindrances to his engagement. He did not care very much for these manifestations, although they automatically developed.