There would give Castiel no greater pleasure than to introduce Dean to some of the siblings created around the same time as him. Cas hadn't informed him before now because the upside outweighs the consequences.

The last thing he wants to do is give Dean a false sense of hope. It is not his intention to shoot down the ideas Dean is brainstorming. There is a moment where Cas realizes between them he creates the awkward silence, and to remedy it he puts himself into Dean's shoes as Sam once suggested. "It seems...there is no harm in trying. You've found the answer for the impossible before." He accepts the outcome whatever it is though he knows Dean doesn't accept change all that well.