Soulbond is a fun new mechanic from Avacyn Restored that lets you pair 2 creatures together, and both get fun new effects. Soulbond says that when the Soulbond guy enters the battlefield under your control, or a different guy enters the battlefield under your control, you can pair the two together if they aren’t paired. But, the thing is, they STAY paired for as long as they’re both on the field under your control (and both of them are creatures). You can’t just break the two of them up because Wingcrafter would be happier in a relationship with Geist of Saint Traft; he’s already with that Delver of Secrets you played on turn 2, and they’re together until death [or exile, or bounce, or Mind Control] do them part.
The headline is slightly simplified, because the other way for creatures to “break up” is if one of them stops being a creature. So if your Inkmoth Nexus is animated, then you cast a Silverblade Paladin and pair them, that bond will only last until the cleanup step, when your Inkmoth goes back to being a plain old land. Now your Paladin is free to pursue other—perhaps longer-lasting—pairs.