Danes ruled over Norway in what the Norwegians call "the 400 year night". Needless to say, they're not well liked. Norwegians used to write Danish until they got their own nation and had the idea of developing their own written standard. A norwegian linguist travelled the length of norway to find the most obscure and un-Danish dialects to create a written standard of Norwegian that was as un-Danish as possible.
And then there is the whole "similarity breeds contempt" thing, which is why we like to shit on the Dutch and the Austrians. It's not real hate, but a constant ribbing and bullying.