It is scientifically proven that all mammals have hair. Mammals that appear to be smooth, like pigs or whales have a fine coating of fur on their bodies, as do humans. Even the smoothest of skin has a small layer of body hair. Although humans as a whole have the smallest amount of hair/fur than any other primate, it is not clear why men have a considerably larger amount of hair than women, evolutionarily speaking that is. However, there are probably a few things going on.....
1. Sexual selection
Believe it or not, once upon a time, men were chosen as sexual partners over their smooth-as-a-baby's-bum counterparts. We can use this theory to at least explain facial hair - which can be likened to the manes of Lions (apparently a definite turn on - the thicker the better!!). Body hair can also be linked to testosterone levels, which may explain why body hair was sought after in a time where our natural instincts decided out sexual partner.
If your dad's hairy, you're probably going to be hairy too. The level of body hair in each individual dramatically changes from one person to the next, and unfortunately, (or fortunately if your dad has a smooth back) this is simply determined by your inherited genes/level of hormones.
Have you ever noticed that the hairier you are the less you get bitten by mosquitos and other insects? Body hair creates natural protection against our flying friends as they simply can't get to your blood! This can be a reason why our great great ancestors in insect-prone areas were naturally hairy, but also why our hairy ancestors survived (and passed their lovely genetics on to us). If they have less chance of getting bitten, they also have less chance of contracting illnesses passed through insects. No malaria for hairy men! Maybe, or maybe not, always wear mosquito protection no matter how hairy you are!
4. The aquatic ape hypothesis
I'm not sure to what extent i believe this theory, or if i believe it at all, but there is a hypothesis out there which tells that humans lost most of their mammalian hair during a phase where prehumans were living in a semiaquatic environment. Yes that's right, women were mermaids and men were pirates. No, the theory goes that whilst living along lakeshores or seashores, women lost most of their hair because they spent more time in the water, whilst the men spent more time on the shore. So start swimming and your hair will disappear.....i think not.