Why do some things just look better in pairs?

My Mum (who is a freelance florist) says that flowers always work best in odd numbers - it's something to do with our minds embracing chaos - it looks more natural.

But sometimes things just look better in pairs. I think a pair can often look more considered than one thing on its own. For example:

 ...Pair of lamps, pair of perspex tables, pair of white ottomans. This room looks so smart. 

Of course other good things also come in pairs...