In the first quatrain the poet categorically refuses to liken his young friend to summer but in the second quatrain he writes: 'But the eternal summer shall not fade' .is the poet contradicting himself?What is the effect of this contradiction?

In this section, Shakespeare is describing the season of summer as a time of extremes and disappointments. He uses his descriptions to contrast the "fair lord" with the summer. The "rough winds" are unwelcome, the length of the summer months are too short. It's too hot, it's too cloudly..... the season in itself is a disappointment. The fair lord, however, has no such imperfections..... he is "more temperate" and his "eternal summer shall not fade". The effect of these lines is to support the speaker's view of the fair lord and his perfection.