All of the quatrains express a single thought in different ways.
Each of the quatrains describes love as constant and never changing. The first says that love isn't love if the feeling changes when the person who is loved changes. The second says that love never changes "it is an ever-fixed mark". It continues this idea of love being constant in the last quatrain when it talks about how love doesn't change over time. He says that a person's looks will change and alter of time, but love does not. It isn't until the final couplet does he comment on this topic of love being constant. This content structure is typical of Shakespeare's sonnets.
