Think of the mind as a body of water, like a pond, or river, or lake. It has a natural flow, and responds to the environment around it with currents and waves and tides. We call these liquid movements of the mind by different names depending on how we experience them — thoughts, moods, emotions, passions, etc. — and when we are meditating we seek to let them all come to stillness. But it's useful to know there is no separation here. These 'objects' (thoughts, emotions, etc) are epiphenomena: the mind's way of explaining its own movements when it cannot see itself fully.
Before we cultivate the concept of anatta we have an egoic self, and the nature of an egoic self is that it tries to generate, organize, and marshal these flows to its own purposes. If the egoic self feels a slight, it may not be enough that a wave of anger rises and falls. That wave may have to be corralled, perpetuated, and put to use to achieve some vengeance. If the egoic self feels the stirrings of love, it may not be enough to let that current flow as it will. The egoic mind may channel and focus that current into romantic pursuit, ardor, even possessiveness and jealousy.
All of this can lead to the stagnation of emotion: the egoic mind forcing the movements of the mind into ruts or patterns that may or may not be useful, functional, or healthy. We all know someone who is constantly angry and constantly looking for new reasons to be angry, because the egoic mind has established that pattern. They have an attachment to anger, and that attachment ties up the natural flows of the mind in that one emotion. When one developed an understanding of anatta, those attachments start to break away (because there is no self to anchor them to) and the energy tied up in them returns to its more natural state.