We're talking about connotations, and the way particular connotations accrue to particular words is unpredictable in advance and often still inexplicable after the fact. We discussed this on the "Connotations" thread.
For example, the definition of the words "rife" and "teeming" is basically identical to "abounding" and a whole set of other words. Yet the first two words have negative connotations. I would complain that your back yard is "rife with mosquitoes" but I would never brag that my back yard is "rife with butterflies." I would criticize the alleys of a Third World city for "teeming with rats," but I would never compliment the promenades of a First World city for "teeming with poodles."
Perhaps since the advent of electronic recording we'll be better able to track the history of words. In the future our descendants may be able to figure out why, in the 21st century, "crap" and "pussy" became acceptable in casual conversation, were frequently heard on TV, and even occasionally popped up in board rooms, whereas "shit" and "cunt" were still considered vulgar.