The Scots have a seperate identity from the English despite being part of the United Kingdom (which ironically ws set up by a scottish king). It's a manifestation of the Roman vs Barbarian inheritance which has coloured european politics since ancient times. I'm not saying the modern scots are barbarians (they're quite a cultured people when they stay off the booze) but they descend from tribes the Romans never conquered. Of course the earlier history of England and Scotland is one of extended conflict, and whilst it may seem strange that this isn't something that's forgotten, these old hostilities can survive in the folk memory for exceedingly long periofds of time.
As with any people that have a regional identity, there will be those who want to establish indepenence on the assumption that life will be more suitable for them - the same motives drive hostiltiies in the middle east and africa for instance (it was also part of the issue in the American Civil War - slavery was something used as a moral rationale by Lincoln). Nationalism can be a heady brew thus many Scots might vote in favour despite any sensible argument against it. Personally I think the only reason this has come about is Alex Salmond wants his name in the history books. What ultimately might happen to the Scots is not really what he wants to hear right now.