I have noticed throughout my life that, whenever I am at a picnic, or a birthday party, or some other outing, adults break into spontaneous celebration when it is time to serve the watermelon. It's the type of celebration that is half genuine and half designed to excite the children about the enormity of the treat they are about to get.
What is the big deal here? Watermelon is in the gourd family; how good could it actually be?
Here's how I see it: Watermelon is difficult to eat and requires a large knife (requires "equipment" would be less of an understatement) to serve. Each bite begins the toilsome exercise of separating the pulp from the seeds. Once separated (and in fact, before), the pulp is all but tasteless. And on top of it all, it is very messy and usually leads to juice running from the mouth area to all clothing and body parts to the south. Watermelon should never be celebrated.
(I know; I know. Please spare me the angry emails about the seedless watermelons you imaginary readers enjoy. They are still hard to serve, tasteless, and messy to eat.)
I think the only reason people buy watermelon is because, when they were children, all the adults cheered whenever it was served leading to the mindless association of watermelon with things that are good. Now, they are the adults, unwittingly perpetrating this cultural anomaly upon another generation of impressionable youths.