Cold sores that are found of the lip are mostly result of viruses. A common virus that can cause this is the Herpes virus which people often get as an STD. For more information about this, follow this link: http://www.ask.com/wiki/Mouth_ulcer#Infection
You can get cold sores through means of direct skin-to-skin contact with someone or something that has the herpes virus. Cold sores on the lips are accompanied by painful blisters, which appear near the area where the virus entered the body.