Did He Tell Her?

We all know the famous Rashi about Shalom Bayit in this week's parshah:
When Sarah heard the angel bless the new couple with a child, she laughed saying,

And Sarah laughed within herself, saying: 'After I am waxed old shall I have pleasure, my lord being old also?' (Bereishit 18:12)

God confronts Avraham with a challenge:

And the LORD said unto Abraham: 'Wherefore did Sarah laugh, saying: Shall I of a surety bear a child, who am old? (verse 13)

Rashi, notes that God changes some of the details of the story. Instead of saying that Sarah thought that Avraham was too old, God only tells Avraham that she considered herself too old. Why did God change the facts? In order to maintain peace within the family. From this Rashi we learn that one may actually change facts in order to maintain peace - mipnei hashalom.
Sounds great. I have one simple question: If Hashem really wanted to keep the peace, why did He say anything at all? Isn't that kind of like picking a scab? Why does God need to interject and rebuke anyone? Why not leave well enough alone, and Avraham and Sarah avoid a fight entirely?

It's really late before Shabbat, so I can't answer the question now. But I have a suggestion based upon the comments of Ramban on this section - which I'll share after Shabbat. Still, it's a good question to discuss over the chicken soup.