Very wide range of risk factors and causes for aphthous ulcers including any sort of physical or chemical irritation, there are probably genetic factors.

Minor vs major vs herpetiform: how big and painful!  HSV is possible but tends to affect lips and produce crusts.

There is some suggestion that iron, Folic Acid and B12 deficiencies can trigger it.

Food triggers: acidic foods such as tomato, citrus. Nuts, chocolate, wheat and spices.

Cinnamon and benzoates – Glasgow study of adults with RAS or Orofacial granulomatosis and other oral mucosal diseases found significantly higher rates of positive patch testing in both groups (70%, cf 60% of controls!), esp food additives (benzoic acid, salicylic acid, tartrazine, glutamic acid, butylated hydroxytoluene, butylated hydroxyanisole, propylene glycol, sorbic acid and sodium metabisulphite), – 41% contact urticaria cf 22% controls – with high rate for benzoate,  Perfumes and flavourings 40.7% overall, vs 9% controls –  of which cinnamaldehyde most important (32.4%).  Chocolate was mentioned specifically but actually only 3.7% positive.  [QJM. 2000 Aug;93(8):507-11. PMID 10924532]

Aphthous ulcers can be a sign of an underlying problem including inflammatory bowel disease, coeliac disease, Behcet’s and PFAPA syndrome but you would expect other signs and symptoms.

The less obvious cause would be cyclic neutropaenia.

Aphthous ulcers can be a lifelong problem although they tend to be less of an issue after teenage years.

Treatment

Apart from Bonjela, Difflam spray, chlorhexidine mouth rinse.  Cholinesalicylate dental gel (not licensed under 16 years).

Steroids: Hydrocortisone dissolving tablets, else a steroid inhaler sprayed in to the mouth or Betametasone soluble tablets as  mouthwash (unlicensed).

BNFc mentions doxycycline rinsed in mouth!

Salt water rinses, applying teabags or Aloe juice directly to the ulcers!?