Thank you for the question, Matt. Hair appears only on mammals, and comes in many forms: hair on the heads of humans, whiskers on cats, bristles in a lion's mane and even quills on a porcupine. It contributes to many functions, including insulation, camouflage, sexual signaling, protection, and sensing the world around us.
In humans, ear and nose hair protects from dust; eyebrows and eyelashes protect against small particles and sunlight; body hair provides insulation. These hairs do not need to grow beyond a certain length in order to execute their function.
Human head hair is different. Hair provides a great deal of insulation (90% of body heat loss occurs through the head); so, up to a point, longer hair equals better insulation. But researchers also believe head hair plays a role in sexual attraction. Long, thick hair is a visible sign of health in the animal, which could explain why longer head hair is often viewed as being sexy.