Some headlights dim over time for several reasons.

First, the bulbs themselves can deteriorate. The filament inside the bulb may wear out or break, reducing the light output. To solve this, replace the bulbs.

Second, dirt and grime can accumulate on the headlight lenses, blocking light. Clean the lenses regularly with a suitable cleaner to restore clarity.

Third, the reflector behind the bulb can get damaged or corroded, affecting light reflection. If this happens, it might require replacing the entire headlight assembly.

Fourth, voltage issues in the vehicle's electrical system can cause headlights to dim. Have the electrical system checked and repaired if needed.

Fifth, age and exposure to the elements can degrade the headlight's components. Using protective coatings or covers can help prevent this.

Remember, maintaining clean and functioning headlights is crucial for safe driving.