One of the current theories for why synesthesia occurs is that areas of the brain with different senses still have neural connections. These are suppressed in most people, are not suppressed in those individuals with synesthesia. Stimulation of one sense can involuntarily cause the stimulation of an area of the brain that deals with another sense, causing the person to feel/see/hear sensations that are not actually there. So while it is an “association” by definition, it is not the sort of association that is made by a person, but it is hardwired into their brain. It is important to remember that to a person with synesthesia, their experience feels just as real as the rest of reality, because to their mind, it is.
Synesthesia is not normally called an illness or disorder because it has practically no negative effects unless you just get really unlucky in what sort of synesthesia you have. It may have some benefits, since the connections between the senses may promote creativity.
-
Change of topic:
Once, I read that people have had synesthesia “given to” them and that synesthetes had synesthesia “taken away from” them through hypnosis (not stage hypnosis, but actual professional hypnosis). For some participants, this did happen. If I remember correctly, they were even put through brain scanners. One of the questions was whether this operates on the same level as actual synesthesia, but for now it does seem to produce the same effects.