When we think of impermanence, we may not think of the impermanence of our happiness. The Buddha himself suffered even after attaining enlightenment. It is inevitable that we go through moments of more happiness than others, more serenity than others, and more suffering than others.
When we find ourselves in peaceful moments, our mindfulness brings us to appreciate them deeply. We are able through mindfulness to touch our serenity, and live in the current moment. Also, we are able to touch the impermanence of our peace. This mindfulness helps us when we are not in times of as much peace. Practicing mindfulness, we recognize the impermanence of our feelings, regardless of what they are. We are bound to have ups and downs, good times and bad times. Being mindful of our emotions and the impermanence of them, we become more accepting of the ups and downs, knowing they pass as they always do.