The same reasons they are faught everywhere - people clashing over their attempts to gain more economic and/or political power. In recent centuries in Europe I guess it would have been due to the number of industrialised (or industrialising) and militarily strong countries seeking to expans and empire build and the clash of their ambitions.
Religious affiliation also played a large part in European wars prior to that - basically with war breaking out between Protestant and Catholic nations. Although, this was essentially still political conflict over the power of Rome rather than people personally fighting it out over differences in doctrine.
One of the reasons why there were so many wars in Europe was because there are so many small countries close together. Lots of countries gained lots wealth in Europe and needed to expand their borders. They were often tripping over each other to get to resources and foreign territories. Like the New World and Africa