Since time immemorial, living entities have been going through many lower species of life, gradually rising through the evolutionary process and, due to some good fortune, receiving a human birth. In the lower species of life the spirit souls are densely covered by the material modes, and thus carnal appetites dominate their lives.
Among the human species, some denounce sensual pleasures and are honored by the world as a saints, yogis, philosophers, scholars, and so on. They experience mental perceptions far superior to gross sensual experiences, and may reach even subtler levels of fine intellect. But even more subtle than the intellect is the spirit soul. So true spirituality, or the real religion of the living entity, means to be situated in self.
Except for the religion of the self, all paths and religions are pseudo-spiritual exercises consisting only of eating, sleeping, mating, and defending oneself from danger. These are the primary activities of the animals. The lower species cannot elevate themselves by executing the religion of the self, or soul. But since human beings are inherently able to practice the religion of the self, some endeavor to reach perfection. Only as a human being can one make such inquiries as “Who am I?” and “Why do the threefold miseries always give me trouble?”
Human life alone affords one the chance to attain unending happiness. In this life one should think, “Although I do not want suffering, it nevertheless comes; although I do not desire death, it forcibly snatches away my life; although I detest old age, when my youth is finished I will surely begin to age; and although I try to be free from disease and disaster, they never leave me alone.”
Although he sees all this suffering, a fool works hard to make his life comfortable, whereas an intelligent person calmly considers his situation and thinks of the best means to end his distress once and for all. When such thoughts become frequent and sincere, his search leads him to inquire into the Absolute Truth. Such a person takes up the path of self-realization. He may have many duties, but because of his previous pious activities such a wise person will execute these duties and at the same time confront the realities of birth, death, old age, and disease.
Ref: Renunciation through Wisdom authored by Srila Prabhupada (Founder – Acharya, ISKCON)