Summer traditionally represents the time in life when we are fully blooming.
Explanation:
Summer traditionally represents the time in life when we are fully blooming. Spring is the virtuous youth. The metaphor he is playing with is the traditional notion that we all live the seasons of man, and that we have the most promise in the spring and are at our hottest (ripest) in the summer.
The main purpose of Shakespeare's Sonnet 18 is embodied in the end couplet: So long as men can breathe or eyes can see, So long lives this and this gives life to thee. The sonneteer's purpose is to make his love's beauty and, by implication, his love for her, eternal.
