Linguistics takes a descriptive approach to language: it tries to explain things as they actually are, not as we wish them to be. When we study language descriptively, we try to find the unconscious rules that people follow when they say things like sentence (1). The schoolbook approach to language is typically prescriptive. It tries to tell you how you should speak and write.
Notice that there is a place for both description and prescription in language study. For example, when adults learn a foreign language, they typically want someone to tell them how to speak, in other words to prescribe a particular set of rules to follow, and expect a teacher or book to set forth those rules. But how do teachers know what rules to prescribe? At some point in time, someone had to describe the language and infer those rules. Prescription, in other words, can only occur after the language has been described, and good prescription depends on adequate description. We obviously don't want to be teaching people the wrong things about language.
In an ideal world, descriptive and prescriptive approaches to language would follow this harmonious relationship: linguists would describe the rules of a language, and pedagogues would use those descriptions to make textbooks to teach language learners. In the real world, however, practitioners of the two approaches often separate themselves into hostile camps. Prescriptivists accuse descriptivists of being anarchists who want to do away with all rules of language. Descriptivists accuse prescriptivists of uninformed bigotry. With each side posting guards at the ramparts to repel the enemy, both tend to ignore the work and concerns of the other. Grammar textbooks used in K-12 education often neglect the findings of linguistics and instead copy outdated, factually incorrect material from older textbooks. For their part, linguists frequently treat prescriptivism as a bad word but fail (with some honorable exceptions) to show how their abstract theorizing is relevant to language teaching.
The conflicts between prescriptivism and descriptivism originates in a difference in focus: scientific study versus teaching. But that difference hardly explains why the two groups are so hostile. Other disciplines don't have a similar divide. High school physics teachers don't scorn the abstruse theorizing of university professors in quantum mechanics or string theory, even if those theories are far beyond the level of high school physics. They take it for granted that there is a continuity between the basic—and simplified—principles taught in introductory classes and the work that cutting-edge research scientists perform. Why is the study of language different?
One reason may be the emotional investment we all have in language. Language is more than a neutral medium for transmitting a message. It has washed over us like a river continually since birth. We use it constantly. It shapes who we are. Think back to your earliest memories. Can you ever remember a time when you were without language? Identity and language twine about each other so tightly that they are impossible to separate. Children of immigrant families, for example, often associate the language of their home with warmth and strong personal connections, with the deepest, private sense of who they are, in contrast to the formal public language of school and the outside world.
Language serves as a symbol of group identity. With the words we use and the way we pronounce them, we send signals to others—conscious and unconscious—about where we come from and how we see ourselves. Children, and adults for that matter, will adopt slang terms to show that they are hip, part of the in crowd. Some people view English as the unifying force of America. According to this perspective, the major thread holding a diverse society together is language. Those who stress this point emphasize the need for immigrants to master English, and sometimes insist that English should be the only language used in public life in the United States.
You don't have to accept this conclusion yourself to see that the choice of language involves deep questions of who we are and how we envision our relationship with society at large. For that reason, pronouncements about language can provoke strong reactions. When someone tells us that the way we use or understand language is inadequate, it's only natural to bristle. A challenge to our language can be tantamount to a challenge to our inner selves. So when disagreements arise over how we use language, the emotional stakes are higher. Over the years, we have developed a strong intuitive sense of what language is. Most of us probably find ourselves much more detached from questions such as, "How did the universe begin?" or "What happens if you travel at the speed of light?" If our assumptions about physics are wrong, we don't take it personally.