If it works, you're with someone who is with you only because you've manipulated him into it. If it doesn't, you gain a reputation as a manipulative head case. What do you mean, you don't want either of those outcomes? Of course you do.
Here's a particularly good tactic. Let's say you're after someone who has had a previous relationship with someone whom we'll call Ralph. Tell him that if he doesn't take up with you, he'll go through life with another Ralph and another Ralph and another Ralph. He's likely to respond that Ralph used the same line, so be prepared to change the subject immediately.