Originally Posted by c1atsite
does anyone know how they even start to begin with?
Sort of. Injury is the most common cause, the abrasion or cut then becomes infected because mouths are very very dirty.
Other causes include sensitivities to things like toothpaste itself (sodium lauryl sulfate is a harsher cleaner than many realize). Various viruses can cause them. Allergies. Fungal infections. Problems with the immune system, both autoimmunity and immunodeficiency. Cancer. And even vitamin deficiencies. A diet in too acidic of foods.
If you have dry mouth, or sleep with your mouth open, that could increase your chance of getting them as your saliva protects your mouth and to a degree controls bacteria levels. There's also dental problems which can be a cause.
A lot of medications can cause them as a side effect, as well.
A very brief and vague run down of causes.
For most of us I'm willing to bet it's just a combination of injury, diet, and maybe some vitamin deficiencies. Unless you get them very often and they're bad, don't worry too much.