Primary causes of dehydration during exercise include inadequate fluid intake, excessive sweat loss, and environmental factors like heat and humidity. When exercising, the body relies on perspiration to regulate temperature, leading to significant fluid losses, especially during intense activities or prolonged sessions. Failure to replenish these lost fluids can result in fatigue, dizziness, and impaired performance. Staying hydrated is crucial before, during, and after exercise to maintain optimal bodily function and prevent dehydration-related symptoms.