Why did the author of sonnet xviii compare beauty a woman to summer?

In Sonnet XVIII, William Shakespeare uses the comparison of a woman's beauty to summer as a means to convey the fleeting and transient nature of both beauty and life itself. Just as summer, with all its vibrant beauty, is destined to fade and give way to autumn and winter, so too does human beauty inevitably decline with the passing of time. The poem's speaker emphasizes the urgency of appreciating and cherishing beauty in the present moment, before it fades and becomes a "summer's lease" that has expired.

By drawing this parallel between a woman's beauty and summer, Shakespeare highlights the bittersweet reality that all things beautiful and pleasurable in life are ultimately impermanent. This reflection on the transience of beauty also serves as a reminder of the mortality and temporality of human existence. The poem thus carries a philosophical message about the need to embrace and celebrate the fleeting moments of beauty and joy in life, while acknowledging the inevitability of change and decay.