Why do they appear?
Aphthae are small wounds that we cause ourselves inside the mouth. How? Well, in most cases when we bite our cheek or our tongue, as a result of using a toothbrush which is too hard, or rubbing with our braces. This does not mean that aphthae will necessarily appear if we bite ourselves, when we brush our teeth or if we wear braces.
To be nervous is also conducive to aphthae. That is why many students have them during exam periods.