One of the main design principles is odd numbers are more pleasing to the eye.  Whether is is landscape design, interior decorating, fine art or scrapbooking, an odd number of items is a general rule to design and layout.

For example, if you were adding decorative brads to a layout, add one brad or three brads (or even five brads) and it will create a more pleasing look then say, two brads.

With your next project, try it out.  Place two decorative elements to a page, then remove one or add another and you will see that it creates a better flow.