In the Five Element system, the Wood of Spring generates the Fire of Summer. During summer, Yang energy expands to its fullest potential.
Fire is the element of the Heart, so the summertime is a wonderful time to work with the energy of the Heart.
In Chinese Medicine, the Heart is also known as the "Emperor", and it is associated with the mind and the spirit, (the Shen). Experiences that affect our Heart, can therefore strongly affect our mind and our mental wellbeing. The virtue of the Heart is Peace, and its acquired emotion is Joy.