Having unrealistic expectations in life can sometimes become a(n) stressor.

Internal

Explanation:

Internal stressors induces thoughts and behavior and often the most common sources of stress. There are certain thoughts and feelings that come from one's psychological expectations and mind which causes stress and make to feel unease .

Having unrealistic expectations in life is one of the example of  internal stressor. When someone thought unrealistic and it does not happen in real life that causes stress on the person.

Some of other example of internal stressor are  putting pressure to be perfect of public speaking, low self esteem, uncertainties and apprehensions.

Hence, the correct answer is "Internal stressor".