Sans apologizes. And he leaves, wisely. Mettaton stands there for a long few moments, just trying to exist, trying to hold onto the precious happiness and brief moment of clarity he had when seeing him. He knows. He knows what Sans means and he knows it isn't a lie and he should be jumping for joy, should have pulled Sans into his arms and kissed him so hard they'd both see stars.

But that's not how it works here. Not now at least. He said I love you to save him and Mettaton wishes it had. He wishes it had been enough to break through the spell and set him free. It may not have saved him now, here, but...

It gives him hope. When this event is over, he knows. He knows it will be his turn to tell Sans the truth and he hopes fervently that nothing will get in the way of that when the time comes.