Most scientists and researchers hold a common theory that attention deficits  occur because sections of the brain do not adequately "gate out" extraneous inputs or stimuli around a person. Thus, someone who experiences ADHD or ADD is usually thought to be absorbing all  sensory inputs with equal weight which results in a sensory overload.

The various causes noted by many scientists include genetic and/or
 environmental  factors such as early exposure to toxins, complications
during pregnancy (such as oxygen deprivation), low birth weight, alcohol
and drug usage during pregnancy, and unusual allergies to food additives.

Research has shown that ADHD and ADD is not caused by too much T.V.,
too much sugar or caffeine, poor home life or poor schooling.  However,
some of those things may llater exacerbate the ADHD and ADD problems.

What do we mean by sensory overload?

Right now you are reading these
 words and can focus your attention
 on what they mean. There may be
cars passing on  the street, some
people talking in another room,
a TV blaring in the next room
and a dishwasher spinning.
Yet you can still focus your
primary attention on reading
these words and can "gate out"
 those other events.

Oak, now imagine that you are
 trying to read these words, but
 the car engines, other peoples'
voices, TV sounds, and
dishwasher noises all come at
you with equal weight. 
It is hard to gate out those 
distractions and you cannot
focus attention upon the words
 and their meaning.
 
That is what people with ADHD
and ADD report:  great difficulty
being able to keep from being 
distracted by outside "chatter".  
Their sensory inputs, all being 
equal  in weight, disturb the
ability to concentrate and focus.
Thus, cognitive overload occurs.