Getting myself into trouble

Why do I do this to myself? I approached my boss about getting some more hours by asking if they were interested in offering ESL classes at the library, and now he wants me to teach two classes during the 6-hour time period I am there on Saturdays instead of one — essentially cutting out my prep time. Turns out they are desperate for an ESL teacher — 32 people are on a waiting list — and so now, suddenly, I feel an enormous amount of pressure to go ahead with it, even though I won’t get any additional pay and will have to prep during my free time. Why? Why?