Have you ever tried decorating a table, shelf or wall with 2 or 4 items and it just felt wrong? Well, believe it or not, there is a neurological reason that it bothers you. Our brain loves patterns. Our eyes are constantly scanning our surroundings to find them, even when we don't realize it. So it is only natural that when our eyes cannot perceive a clear visual pattern it bothers us. 3 is the lowest number that can create a pattern so our brain just loves it! When we encounter items in odd numbers it forces our eyes to move around which leads to a more visually interesting experience! So also consider decorating in 5, 7s, 9s, etc.
When you are decorating keep the Rule of 3s in mind. Your brain will thank you!