Why do some people lack basic social skills?
And if they don't have any - why don't they watch and learn.
In my experience the following set of species are the most irritating -
1) The advisor - Shall I add that "unsolicited" precedes advisor? They are usually the unassuming kinds who seem very harmless but with one mighty lash of their tongue they could spoil your entire day. They can dispense expert advice (in their opinion) for anything and everything under the sun! From "why don't you give your child some honey for the cough?" - I just did that lady! to "You shouldn't wear that dress/shirt/top/whatever..." - Did I even ask your opinion. And no they just don't get that "look" you give them.
2) The Ms/Mr take-you-for-granted - These kinds think they can use you for anything and everything - without asking. This could include using your car to make a "quick" trip to the supermarket, borrowing your iron/hair dryer/mixer and not returning it till you go and ask for it after 10 days, or sending their kid over to play with yours every afternoon just when you are about to nap! Ouch!
3) The nosey parker - This species relishes any opportunity to head into your house and update their current affairs knowledge. They know who in the neighbourhood is planning to run off with whom, whose geyser is not working, whose maid steals and who went shopping. They are often found stuck to the keyhole on their main door!
4) The know-it-all - This species considers themselves the Encyclopedia Britannica's of the world. For everything you say they will have something more to add. They always want to have the last word. No one measures up to their strict criteria of "being-perfect" and "knowledgeable". They never lose the opportunity to show off their gathered expertise.