SB 9.24.57

na hy asya janmano hetuḥ
 karmaṇo vā mahīpate
ātma-māyāṁ vineśasya
 parasya draṣṭur ātmanaḥ
Word for word: 
na — not; hi — indeed; asya — of Him (the Supreme Personality of Godhead); janmanaḥ — of the appearance, or taking birth; hetuḥ — there is any cause; karmaṇaḥ — or for acting; — either; mahīpate — O King (Mahārāja Parīkṣit); ātma-māyām — His supreme compassion for the fallen souls; vinā — without; īśasya — of the supreme controller; parasya — of the Personality of Godhead, who is beyond the material world; draṣṭuḥ — of the Supersoul, who witnesses everyone’s activities; ātmanaḥ — of the Supersoul of everyone.
Translation: 
O King, Mahārāja Parīkṣit, but for the Lord’s personal desire, there is no cause for His appearance, disappearance or activities. As the Supersoul, He knows everything. Consequently there is no cause that affects Him, not even the results of fruitive activities.
Purport: 

This verse points out the difference between the Supreme Personality of Godhead and an ordinary living being. An ordinary living being receives a particular type of body according to his past activities (karmaṇā daiva-netreṇa jantur dehopapattaye). A living being is never independent and can never appear independently. Rather, one is forced to accept a body imposed upon him by māyā according to his past karma. As explained in Bhagavad-gītā (18.61), yantrārūḍhāni māyayā. The body is a kind of machine created and offered to the living entity by the material energy under the direction of the Supreme Personality of Godhead. Therefore the living entity must accept a particular type of body awarded to him by māyā, the material energy, according to his karma. One cannot independently say, “Give me a body like this” or “Give me a body like that.” One must accept whatever body is offered by the material energy. This is the position of the ordinary living being.

When Kṛṣṇa descends, however, He does so out of His merciful compassion for the fallen souls. As the Lord says in Bhagavad-gītā (4.8):

paritrāṇāya sādhūnāṁ
 vināśāya ca duṣkṛtām
dharma-saṁsthāpanārthāya
 sambhavāmi yuge yuge

“To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear millennium after millennium.” The Supreme Lord is not forced to appear. Indeed, no one can subject Him to force, for He is the Supreme Personality of Godhead. Everyone is under His control, and He is not under the control of anyone else. Foolish people who because of a poor fund of knowledge think that one can equal Kṛṣṇa or become Kṛṣṇa are condemned in every way. No one can equal or surpass Kṛṣṇa, who is therefore described as asamordhva. According to the Viśva-kośa dictionary, the word māyā is used in the sense of “false pride” and also in the sense of “compassion.” For an ordinary living being, the body in which he appears is his punishment. As the Lord says in Bhagavad-gītā (7.14), daivī hy eṣā guṇamayī mama māyā duratyayā: “This divine energy of Mine, consisting of the three modes of material nature, is difficult to overcome.” But when Kṛṣṇa comes the word māyā refers to His compassion or mercy upon the devotees and fallen souls. By His potency, the Lord can deliver everyone, whether sinful or pious.