When you were reading Shakespeare's Sonnet 73 for the first time, you'd be forgiven for not knowing this was a love poem. After all, it takes 13 lines for Big Willy to finally let the cat out of the bag. But doesn't that kind of make the love-theme all the more powerful? Think about it: if the entire poem up to that point has been dealing with basically the opposite of love (you know, decay, destruction, coldness, death, etc.), doesn't that make it all the more amazing that love refuses to knuckle under? In this poem, love's fashionably late entrance lets it become the life of the party.
The speaker waits until line 13 to introduce the theme of love (a) to make it more of a surprise, and (b) to make love seem more impressive by showing what it's up against.
The speaker thinks love can triumph over many emotional obstacles, but it still is limited by mortality and nature.