sweetness and light

Ostentatious amiability and friendliness, as in One day she has a temper tantrum, the next day she's all sweetness and light. This phrase was coined by Jonathan Swift in his Battle of the Books (1704), where it referred literally to the products of bees: honey and light from beeswax candles. But in Matthew Arnold's Culture and Anarchy (1869), the term meant "beauty and intelligence." In the 20th century, however, it was applied to personal qualities of friendliness and courtesy and to the general pleasantness of a situation, as in Working with him isn't all sweetness and light, you know. Today it is generally used ironically, indicating lack of trust in a person's seeming friendliness or for a difficult situation.

The noun has one meaning:

Meaning #1: a mild reasonableness