Nattering Nabob Of Nubbiness

I love the autumn.  Every year I look forward to taking a sweater out of my closet and wearing it on a cool fall day.  And every year, when I do so, I ask the same question:  What in the heck makes my sweaters get nubby?

You know what I mean, I think.  You have a nice woolen sweater that’s warm and soft and perfect for the autumn weather.  You wear it, and wear it, and then one day you notice these tiny woolen stubs that have sprouted up from the sweater, likes eyes on an aging  potato or zits on a greasy teenager’s face.  They’re unsightly, and they’re irritating, as you try to carefully pick them off, one by one.  But we all know that once a sweater crosses the nubbiness threshold, it’s got one foot in the lamb’s wool grave.  The next time you turn around, there will be a few new ones to give you that unpleasantly knobby, senior citizen look.

Can anyone tell me what causes sweater nubs?  And, equally important, is there anything I can do shield my favorite sweaters from an unwanted, knobbly fate?