One of the ways to see how our feelings affect us is to observe our heart rhythms. Feelings that are unpleasant and intense such as anger, frustration, angry etc., show up as uneven and jagged heart rhythms. In contrast, when we feel positive, appreciative, confident, secure etc., our heart rhythms are smooth & even.
These heart rhythms relate to how are sympathetic and parasympathetic nervous systems are working together. Generally these two branches of our CNS are out of synch and are working like having one foot on the gas pedal and one foot on the brake. When they are in synch, they manifest by being smooth and even. This is accomplished by combining our physical effort of belly breathing with our positive thinking. 
Synchronicity achieved by combing belly breathing with positive thinking is superior to just relaxing (which is helpful but could be so much more). With HeartMath, you learn to take control of your emotions and as you become skilled at it, you increase your level of well-being, become more self confident and satisfied in yourself and in your relationships. 
The heart is involved in how we feel & how well the brain integrates information to various parts of itself.