This behaviour may stem from any or all of the following:
Lack of imagination.
Never seen pictures of a road accident.
Someone who only managed to pass his/her driving test by giving the examiner a blow job.
Having an extra chromosome.
Loneliness or pathological nosiness
Drink or drugs.
Being born with somekind of twat gene.
Driver is busy trying to figure out what the indicator lever is for.
A guy that is so dammed horny that he just can't wait to get to his mothers house.
Many female tailgaters like to put a sign in the rear window of their car stating that there is a 'Baby on Board'.
Tailgating may lead to a car fuck
It was pitch black on that road, when I turned my car lights back on I observed that the tailgater had dropped back to reasonable distance.
"fold up your old lady's tailgate, it's about to fall off".
"woke up after a long night of drinking and her tailgate was crushing my penis".
"judging by her Mom's tailgate, she's gonna have one too someday"!