Summer is paired with Fire in the five phase system, the phase of maximum yang, read as the point in the year's cycle where the rising energy of spring reaches its full expression: warmth, brightness, and outward radiating activity at their most intense before the cycle turns back toward contraction in autumn. Where spring's Wood energy was described as directed growth, bending and straightening, Fire's defining quality is upward, radiating movement, the classical image being flame itself, which naturally rises and spreads rather than holding a fixed shape. In the body, Fire and summer are paired with the heart and its partner organ the small intestine, and with the emotion joy, again read within the tradition as the season's natural energetic signature expressed through a specific organ system rather than simply a pleasant feeling; classical Chinese medicine associates excessive or deficient Fire activity with disturbances of the heart and, by extension, of sleep and mental clarity, since the heart in this framework is understood to house shen, the mind or spirit.
In the generating cycle, Fire produces Earth, commonly explained through the image of fire's ash returning to and enriching the soil, so summer is read as building toward the Earth centered pivot that either follows it directly or sits at the turn of every season, depending on which version of the scheme a given text follows.
Sources Astrology and Cosmology in Early China: Conforming Earth to HeavenDavid W. Pankenier with The Foundations of Chinese Medicine: A Comprehensive TextGiovanni Maciocia