How would you honestly feel if your significant other volunteered to donate a kidney?

For the sake of this discussion you are married with kids.

So lets just say you found out that your spouse had volunteered one of their kidneys to their father who is in kidney failure. It is not yet needed and matching has not yet happened but the possibility of it happening is very real.

How do you react? With anger? With Joy? Would you try to discourage them or would you encourage them?
What do you honestly think your reaction would be?

Rather than taint the discussion I will wait to add what I would do later down the thread.

Many thanks to all who contribute their thoughts. This is actually really playing out in my life.