This from the same web page as the picture came from:
Where does navel fluff come from?
Michael Biesecker wrote an interesting article on navel fluff in the 19/4/95 edition of Technician, accessed via North Carolina State University library. In it he discusses the widely held belief that navel fluff forms when very tiny pieces of fibre break off the inside of clothing. These tiny fibres gather in the belly button and amalgamate into balls of lint.
He observes that the colour of navel fluff varies amongst different people, and that those who habitually wear clothes of a similar colour tend to produce fluff related to that colour. However, those who wear a variety of colours usually end up with fluff of a grayish blue colour similar to the lint found in the lint filters of clothes driers. This colour is most likely an average of all clothing colours worn.
Those with hairy stomachs tend to generate more fluff, as abdominal hair is alleged to assist with dislodging fibres from clothes then collecting and channelling them into the navel. Also those with larger bellies often experience greater volumes of fluff - possibly due the tendency of large stomachs to possess deeper navels, thus a larger space for the lint to lodge in.
But how does it accumulate in the navel? Dr Donald E. Smith remarks that navels may possess a moist and sticky secretion that catches whatever lands nearby. On the other hand, Dr Bhupendar S. Gupta, whose doctorate is in the study of textile fibres, attributes the accumulation of navel fluff to the stomach's "microclimate" - where the flow of air between clothing and the abdomen carries small lint particles that get lodged in the navel.
Probably the best investigation into navel fluff was conducted by Dr Karl Kruszelnicki of the University of Sydney. He devotes a whole chapter to it in his popular science book "Q&A With Dr K" (Harper Collins Publishers 2001). The role of abdominal hair in dislodging and channelling clothing fibres is confirmed, but he also suggests the type of washing machine can also play a role. Apparently top-loading machines are not as gentle as front-loaders, leading to greater quantities of dislodged fibres, many of which remain in the clothing and cause greater accumulation in the navel. He also found that a well developed "snail-trail" - hair connecting the pubic hair to the navel - also encourages lint in the belly.
So, maybe those who no longer suffer from it have changed their washing machine...?