Human beings, including your “self”, exist as part of a material world. This “world”, which in this usage of the word represents the entire universe, is governed by the immutable laws of physics, chemistry, and biology. Your existence as an individual human is completely dependent upon these facts. You are an evolutionary animal with specific needs, such as food, water, shelter, and companionship. And however much you may wish it, these biological requirements of life cannot be bypassed, eliminated, or ignored. If you wish to continue living as a human being in the world, you will have to identify these laws of nature, these facts of reality, and meet their requirements. The way you do this is by employing the human faculty of rational thought, which means exploration, investigation, experimentation, logical analysis, concept formation, definition, and proof.
Man, unlike other animals whose behaviors are pre-programmed by evolutionary selection, must use his rational faculties, his mind, to discover the requirements of reality for his continued existence, as well as the means by which to achieve them. Not only that, but every man must determine for himself what will be the purpose of his existence. “Why am I here?” and “What should I do?” are questions that do not trouble the lower animals. They do not even arise, because these questions are dependent upon a long chain of complex mental abstractions, starting from the perceptual level of direct sensory experience and working up through an actively discovered, fact based, scientific understanding of reality, and culminating in the supreme concept of “I am”, which means the recognition of oneself and every other man, as a separate, independent, freewill being that can and must choose between contrasting values and various courses of action.
This requirement of human life, that each individual man must choose his values, his purpose, and his means for attaining happiness is what sets man apart from the lower animals. And it requires him to think rationally if he wishes to survive in the world and achieve happiness in this life.