The Lord says in Bhagavad-gītā (4.7) :

yadā yadā hi dharmasya glānir bhavati bhārata abhyutthānam adharmasya tadātmānaṁ sṛjāmy aham

“Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion — at that time I descend Myself.” Thus it is not an imagination but a fact that the Supreme Personality of Godhead, by His sweet will, appears in different incarnations, such as Matsya, Kūrma, Varāha, Nṛsiṁha, Vāmana, Paraśurāma, Rāmacandra, Balarāma, Buddha and many other forms. Devotees are always eager to see one of the Lord’s innumerable forms. It is said that just as no one can count how many waves there are in the sea, no one can count the forms of the Lord. This does not mean, however, that anyone can claim to be a form of the Lord and be acceptable as an incarnation. The incarnation of the Supreme Personality of Godhead must be accepted in terms of the descriptions found in the śāstras. Lord Brahmā is eager to see the incarnation of the Lord, or the original source of all incarnations; he is not eager to see an imposter. The incarnation’s activities are proof of His identity. All the incarnations described in the śāstras act wonderfully ( keśava dhṛta-mīna-śarīra jaya jagadīśa hare ). It is only by the personal sweet will of the Supreme Personality of Godhead that He appears and disappears, and only fortunate devotees can expect to see Him face to face.