In plain words
Karma is not the basis of God's descent. Rather, the root cause of it is the sins the wicked commit and the merit the good earn. The Narasiṃha Avatāra came because of the Bhakti and surrender Prahlāda attained and the evil qualities Hiraṇyakaśipu indulged in. The cause of God's Avatāra is the feelings of those who live in the world, not karma.
Crops wait in many ways for water. But these plants do not grow until the clouds come. Because they think only of water all the time and live in the feeling of depending on water alone, the clouds themselves come down and nourish the crops. In the same way, to nourish and protect devotees and to establish Dharma, God comes down. That is called avataraṇa. It means coming down. Leaving His exalted state, to protect people and to make them happy, He keeps coming down. People are not trying to recognise such sacred secrets.
There is nothing in this world greater than Bhakti. Once Nārada, teaching his disciples, said there is nothing greater than Bhakti. Among them a student rose: ‘Master! In this world made of the five elements, the earth is very large, isn't it? Is Bhakti greater than this earth?’ he said. ‘True! Water has swallowed three parts of this earth, large as it is. Now which is greater, earth or water?’ asked Nārada. The disciple said water is greater. But the sage Agastya drank up this water in one gulp! The disciple said Agastya was the great one. ‘Great as he is, Agastya is like a little star in the sky,’ said Nārada. ‘The sky itself is great, Swami,’ said the disciple. ‘But when He came as the Vāmana Avatāra, Vāmana asked for three paces of land. With one pace He took the earth; with the second step He took the sky. When He took the sky with a single step, which is greater, God's foot or the sky?’ asked Nārada. When even God's foot is so great, God Himself is the great one, he said. ‘A devotee binds this great God in his heart. So which is greater, God or the devotee's heart?’ he asked. Bhakti is greater, said the boy.
There is no supreme medicine greater than Bhakti. Bhakti is what cures every illness belonging to this world. There are no Vedas and Śāstras greater than this Bhakti.