The Shadow, who presumably hasn’t watched the last decade of Supernatural and therefore doesn’t know how frequently Castiel dies, decided that a dead Cas sooner rather than later was more appealing than dead Cas eventually.
Desperate to save Jack from The Empty, Castiel offered himself to the Shadow instead.
When the Shadow raised that point that Cas would end up in The Empty someday anyway and therefore there wasn’t much incentive to make a deal, Cas reminded it that he could live for eons longer, which means the Shadow staying awake for eons.