One of the main reasons would be enamel thinning, As the enamel layer of the teeth starts wearing down and thinning itself due to chewing, the yellow dentin is exposed making the teeth look yellower, other factors are faulty diet-consumption of coffee, beverages.
Or use of any medication like doxycycline or tetracycline.
Next Steps
Brush your teeth at least twice daily, floss at least once daily to remove plaque, and rinse with an antiseptic mouthwash at least once a day to kill bacteria that cause plaque. Use a whitening toothpaste (once or twice a week only) to remove surface stains and prevent yellowing