There seem to be several
inter-dependent factors at play in causing
addiction in a person. Every addiction starts with some sort of
gratification. That gratification can come in the form of physical
pleasure, as is with
sex or
eating, drawing the attention of others, whether positively or negatively, as is with on-line
communication, a sense of increase in one's
self-worth/
self-esteem, as might be the case with posting to
Everything or any other
physical/
psychological stimuli that result in positive gratification.
Addiction works as a non-linear system. The more a person induces the gratification-generating stimulus, the less gratifying it becomes as the brain learns to habituate its response to it. In trying to re-experience that stimulus at its original full impact, the person is driven to re-induce the stimulus. The decline in gratification keeps at a loosely direct proportion to the frequency that the stimulus is induced.
The evolutional value of the addiction mechanism is clear -- anything that drives a species to eat more and mate more increases that species replicative value. Those that only eat as much as they need aren't going to survive as well as those that keep eating as much as they can, regardless of whether they produce pleasure out of it or not. More directly, those that reproduce compulsively are going to spread their genes much more effectively than those that only do for as long as it provides them with pleasure.