As long as a living entity wants to fulfill various types of material desire, he must continuously change from one body to accept another. Śrīla Viśvanātha Cakravartī Ṭhākura explains that as a small piece of grass falls in a river and is tossed about with different types of wood and tree branches, the living entity floats in the ocean of material existence and is dashed and tossed amidst material conditions. This is called the struggle for existence. One kind of fruitive activity causes the living being to take one form of body, and because of actions performed in that body, another body is created. One must therefore stop these material activities, and the chance to do so is given in the human form of life. Specifically, our energy to act should be engaged in the service of the Lord, for then materialistic activities will automatically stop. One must fulfill one’s desires by surrendering unto the Supreme Lord, for He knows how to fulfill them. Even though one may have material desires, one should therefore engage in the devotional service of the Lord. That will purify one’s struggle for existence.

akāmaḥ sarva-kāmo vā
 mokṣa-kāma udāra-dhīḥ
tīvreṇa bhakti-yogena
 yajeta puruṣaṁ param

“A person who has broader intelligence, whether he be full of all material desire, without any material desire, or desiring liberation, must by all means worship the supreme whole, the Personality of Godhead.” (Bhāg. 2.3.10)

anyābhilāṣitā-śūnyaṁ
 jñāna-karmādy-anāvṛtam
ānukūlyena kṛṣṇānu-
 śīlanaṁ bhaktir uttamā

“One should render transcendental loving service to the Supreme Lord Kṛṣṇa favorably and without desire for material profit or gain through fruitive activities or philosophical speculation. That is called pure devotional service.” (Bhakti-rasāmṛta-sindhu 1.1.11)