The mind is rooted in physical reality. For this reason it is susceptible to all kinds of turmoil and disruption. The food we eat, the state of our health, the pace of our lives all impact on the quality of our thoughts.
We have a choice, however, to be taken on the roller coaster if each thought or collection of thoughts, or we can rise above with the realisation that we are more than our thoughts. We are the observer of our thoughts too. We can watch them as they come and go just like they always do. We can take a step back, bring our awareness to the breath becoming witness to our thoughts without belonging to them or letting them define us. Through yoga we cultivate and strengthen our awareness of the activity of our minds. When we strengthen this ability, we don't need to control our minds activity. We don't need to "calm down" or "think positive" or "look on the bright side". When we rise above the attachment to our thoughts we know that each thought can pass and we, the observer, rooted in stillness will continue to watch and practice preace.