Internal Conflicts and Psychological Harm
An example of an approach-approach conflict is being forced to choose between either one highly preferred treat or another highly preferred treat. Obviously, the result is pleasant no matter what.
An avoid-avoid conflict is the opposite - a forced choice between two unpleasant alternatives. This is quite stressful because either way, the result is bad.
Approach-avoid is a choice between either having or not having something which is both desired and unwanted. An example might be a hungry person who will get a severe punishment for stealing food choosing whether or not to steal the food.
Both avoid-avoid and approach-avoid involve something unpleasant regardless of what you choose, since approach-avoid results in deprivation if you choose not to take the option and something unpleasant if you take it. As a result, both are psychologically harmful. The effects are greater with greater frequency and severity of such choices.
Often, it seems like people assume an approach-avoid conflict is generally with the desire to approach being internal (as in the example of the hungry person). They seem to recognize that the avoid can be internal or external, but not so much the approach. This may be why people assume a behavioral treatment using only positive measures cannot be harmful. However, it can, because it is possible to have an external approach and internal avoid in such a program.
For example, some autistics are internally motivated to avoid eye contact because it is painful. If they are being rewarded for making eye contact, this can cause an approach-avoid conflict.