Ruffs are the fluffy white loopy things that go around a person's neck and usually make a person's head look like a lion's mane. How did ruffs become fashionable and then went out of fashion?
It's hard to say why fashion changes, but ruffs was originally a French fashion that spread to the various countries of Europe.
They may have started as a small drawstring collars in early 1500s on the chemise to protect the upper body garment from soiling.
Portrait paintings from the era gives a good illustration of the evolving fashion, from small collars in the mid 1500s to enormous contraptions towards the year 1600. Obviously only the very rich could afford elaborate ruffs of costly materials.
Ruffs remained fashionable the longest in Spain, in the Netherlands and the parts of Italy under Spanish control. In the 1630s they were outlawed among many other items of luxury by Gaspar de Guzmán,the de facto ruler of Spain 1622-1640, during the reign of Philip IV.
The following is just a speculation; I have nothing to collaborate it with.
First please take a look at this question about wigs. It attributes the fashion of wearing wigs and powdering one's face that appeared in 16th century to the epidemic of syphilis.
Notice now that the ruffs appeared roughly at the same time or slightly earlier, and they started as relatively modest collars that covered one's neck. Is it possible that their function was analogous to that of wigs and powdered face? As the wigs would cover up the signs of the 2nd stage of syphilis on one's head and power would cover the rash on one's face the ruff would cover the sign on one's neck. Just powdering the neck would be less effective than powdering the face because the movements of flexible neck would partially remove the powder.
That doesn't imply that the wearer of the ruff necessarily had syphilis: ruffs, as well as wigs and powder, quickly became the socially required fashion that would let the one who needs them for cover-up blend in.
This explanation is just a speculation, of course, and is has a couple of gaping holes, such as numerous portraits with wigs but without ruffs and vice versa.