My front teeth ache, I have no cavities, no diseases of the mouth, and have been using a night guard. What could it be?

Many things. Without an examination any suggestion is just a shot in the dark. It could be trauma from when you were 15 years old, it could be continued trauma from your bite, it could be dens in dente. It is impossible to say over the internet.
Pressure. The night guard may be exerting too much pressure on those teeth. Have it looked at and adjusted by your dentist.
Your are clenching. Try to find a neuromuscular dentist in your area to fabricate you physiologic orthotic to relax your muscles and help you with your clenching. Or have your dentist adjust your splint to does not have a contact with your opposing teeth on the front.
Many things. Ill fitting night guard, malocclusion (bad bite), cracked teeth, gum disease, just to name a few. This is not a question that can be answered over the computer, please call your dentist for an examination and for help.

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