The deal with Iran is far from perfect. It is, however, the best we could have reasonably hoped to get. Repudiating it tells Iran, "It's OK for you to restart your nuclear weapons program." Even you should should see that this is not a good thing.

Another point is that any other country would be leery of making a nuclear non-proliferation pact with the US, since the American president may repudiate it on a whim. And "whim" is the proper word to use in this instance.