Best Answer:
All of the quatrains express a single thought in different ways.
Step-by-step explanation:
In the first quatrain, the speaker asserts that true love is constant and enduring, regardless of external circumstances. In the second quatrain, he compares love to a fixed star that guides sailors even in stormy weather. In the third quatrain, he reiterates that love is not subject to the ravages of time.
All three quatrains develop the same central idea: that true love is eternal and unchanging. The speaker uses different images and metaphors to convey this idea, but he never wavers from his main point.
The last two lines of the poem (the couplet) serve to summarize and reinforce the speaker's argument. He states that if his assertion about the nature of true love is proven wrong, then he has never truly loved himself and no one else has ever truly loved either.