In Mrs. Frisby and the Rats of NIMH, dissatisfaction is the feeling that propels the whole plot forward. Here's the thing: the rats could have covered their heads with their blankets and ignored those nagging feelings that something is dreadfully wrong in their lives. But does that sounds like the rats we know from this book? Not in the slightest. After their escape from NIMH, the rats experience feelings of dissatisfaction when they begin to understand both that they have been changed forever and that their old way of living is based on stealing from others. And that dissatisfaction drives them to take charge of their lives.
Civilization is pretty tricky in this novel—sometimes it's positive, but sometimes it's the very thing that causes characters to hurt one another the most and to feel the most dissatisfied with life.
O'Brien uses dissatisfaction as a way to teach us something about the character in his novel: the very best characters are the ones who use dissatisfaction to their own benefit.