After giving my girlfriend a bunch of flowers, I was told never to give her an even number of flowers again. Why?

It is a Czech tradition that even numbers of flowers are given only at funerals for the deceased. All other bouquets should have an odd number of flowers. Where this tradition comes from is anyone's guess, but since the same rule applies in many European countries, it is likely to have arisen long in the past when numerology still held sway.