There are four main causes of this common nail condition. Which category lines up with your experience?
Have you injured your toenail? If so, the damaged portion my not grow in the same way that it did prior to the incident. Family traits are relevant as well. Some people have nails that are curvier than others—the result may be recurrent ingrown toenails.
While these things are unavoidable, the other two reasons are within your control. Tight footwear encourages this condition, so make sure that your shoes fit well and that they have plenty of room for your toes. Cutting the nail on a curve also increases your risk. Instead, cut them straight across and file any sharp edges.