The answer to this question lies in our genes, specifically in a gene called MC1R, which is responsible for melanin in our bodies. Melanin is the pigment that determines our hair, skin and eye color. There are two main types of melanin: eumelanin (dark) and pheomelanin (red).
People with red hair have a mutation in the MC1R gene that results in more pheomelanin being produced. Instead of a mixture of eumelanin and pheomelanin, which most people have, pheomelanin dominates in people with red hair, resulting in the characteristic red hair color.
Interestingly, the MC1R gene is recessive. This means that a person needs two copies of this mutated gene - one from each parent - to have red hair. Therefore, red hair is relatively rare and occurs in only about 1-2% of the world's population.
Another fascinating detail: people with red hair are often more sensitive to pain and require different amounts of anesthetics, which is also linked to the MC1R gene.
So, there you have it: a genetic anomaly that causes a whole range of fascinating physical traits and sensitivities!