Why do people give flowers?

When you are ill? Leave a job? When you die? Yes to all of these, but, sometimes they are given when you love someone (but strangely the sentiment is lost if bought from a 24hr garage). Anyway, it was Dites' last day at work for the summer and she was presented with a bunch of flowers (she will not be returning in September). So, all say 'Aaahh!'.