Sharon Salzberg writes in The Buddha Taught One Thing Only,

There are times in our lives when we wish we could change the ending of the story. Sometimes we lose what we care about, we are separated from those we love, our bodies fail us as we get older, we feel helpless or hurt, or our lives just seem to be slipping away. These are all aspects of dukkha, one of the principal teachings of the Buddha. Dukkha means suffering, discontent, unsatisfactoriness, hollowness, change.

The Buddha said, “I teach one thing and one thing only: suffering and the end of suffering.” Suffering in his teaching does not necessarily mean grave physical pain, but rather the mental suffering we undergo when our tendency to hold onto pleasure encounters the fleeting nature of life, and our experiences become unsatisfying and ungovernable.