Why did Leif Erikson sail to Newfoundland?

Answer 1

There are two versions. In the first, he was already aware of it's existence, because it had been spotted years before by a man named Bjarni Herjolfs. In the second, he was sailing for Greenland, but sailed too far south, and ended up in North America.

Answer 2

Vikings loved the idea of new lands and discoveries. He probably wasn't the first, people had probably sailed west before, maybe people from the Greenland colonies, but rumours of a land to the west must be tempting to a man looking for some sort of eternal recognition. So, he took a load of blokes on a boat to find it. This is only an assumption based on the general knowledge of historical Viking behaviour, but as there are very few reliable sources or accounts, about him, it's probably reasonably close.