How we do battle with ourselves: internal conflicts
One of the most important sources of subpersonalities is the way in which we can have inner conflicts - that is, conflicts between two or more of our subpersonalities.
To see how complex this can become, consider for example this quotation from Barry Stevens in Person to Person:
Then I was told things, and I became two people: the little girl who said how terrible it was that the boys had a fire going in the lot next door where they were roasting apples (which was what the women said) - and the little girl who, when the boys were called by their mothers to go to the store, ran out and tended the fire and the apples and loved doing it. So then there were two of I.