This Sonnet focuses on a common theme in many of Shakespeare's sonnets, which is the inevitable and relentless power of time to destroy and take away what is dearest to us. Note how the first line focuses on "Time's foul hand" that defaces everything in the world. Time here is personified as a kind of arrogant destroyer that ruins all in its path. Nothing is free from its malign influence: "lofty towers" are razed to the ground and even the most powerful of states are subject to decay. The overwhelming and tragic conclusion of all that he sees is expressed here:
Ruin hath taught me thus to ruminate,
That Time will come and take my love away.
Notice the word play on how "Ruin" leads the speaker to "ruminate." The ruin that he sees evidence of everywhere he looks, wrought by time, leads him to the shocking conclusion that "Time," just as it destroys everythign else, will also take the speaker's love away from him. The last couplet shows how upsetting and disturbing this thought is to the speaker:
This thought is as a death, which cannot choose
But weep to have that which it fears to lose.
The realisation is "as death," which ironically makes the thought of losing the person that you love so terrible that you weep even when you have them, because you know that eventually they will be snatched by Time.