Have you ever stayed up too late at night watching TV, and hit that point where it's all half-hour infomercials with scary people trying to sell you something that you don't really need? We don't mean to reduce the work of Shakespeare to a cheesy sales pitch…no wait, that's exactly what we mean to do.
In order to get someone to buy what you're selling, you have to start by convincing him that he really needs it. So back to that infomercial. Say they're trying to sell a big jar of eyebrow wax. First you need to get people worried about their eyebrows, by finding the right line: "Have you noticed that your eyebrows look like caterpillars?" or something like that. Then, when they're all freaked out, you give them a solution: a big jar of eyebrow wax!
The same thing is happening in Sonnet 2. The speaker presents a problem (the young man is going to get old) and then offers a solution (having kids). He's selling an idea, so his strategy is the same one you'd use for a sales pitch. He exaggerates the problem, wrinkles turn into "trenches" (line 2). Then he makes his solution sound really great, promising that having a child will be like being born again: "This were to be new made when thou art old" (line13). Kids become a sort of miracle anti-aging treatment. It's a classic sales trick, promising people the world for only a few easy payments of $19.99.