Although some scientists, like the one in this link, disagree, the general argument is that the countries of Northern Europe have milder climates than those of similar latitudes in Asia and the Americas because of the Gulf Stream current. This current is said to bring warmth to Europe, preventing it from having winters that are as cold as those elsewhere.
The Gulf Stream is a major current that runs in a clockwise fashion from the Gulf of Mexico, up the Atlantic Coast of the United States, and then across the Atlantic to Europe. The Gulf Stream is made up of very warm water. It brings that water to the northern latitudes of Europe. The Gulf Stream brings warm and humid air to the countries of much of Northern Europe. By doing so, it allows those countries to have milder climates than those of countries at similar latitudes in other parts of the world. As this link tells us, the Gulf Stream
is responsible for the mild climate of western Europe, which is located at a much higher latitude than most of New England, but experiences much milder weather.