The poet makes his point clear from line 1: true love always perseveres, despite any obstacles that may arise. He goes on to define love by what it doesn’t do, claiming that it stays constant, even though people and circumstances may change. Love never dies, even when someone tries to destroy it. Rather than being something that comes and goes, love is eternal and unchanging – so much so that the poet compares it to the North Star, which never moves in the sky and guides lost ships home. This metaphorical star is mysterious and perhaps incomprehensible, even though we can chart its location.
Moving on to a new image, love isn’t at the beck and call of time (or time’s consequences, age and death); mortality isn’t an issue for true love, which doesn’t fade even when youth and beauty disappear. Love doesn’t change as the days go by; rather, it remains strong until the lover’s dying day (or beyond…chew on that for a while).
Finally, the poet stakes his own reputation on this definition, boldly claiming that if anyone can prove him wrong, he’ll eat his words. That is to say, if this idea of love turns out to be wrong, then he’ll take back everything he wrote and it’ll be as though it never existed. Furthermore, if this specific portrayal of love is somehow proved to be the wrong one, then nobody, as far as the poet is concerned, has ever loved at all.