The first has to do with the fact that Norse myths were codified during the Viking era: 780 1070. This gives the Norsemen many centuries to become exposed to the Greek (or Roman) myths. The Vikings did travel as far east as the Caspian sea, which is further east than both Italy and Greece. The Norse myths were fashioned after the fall of the Roman empire. During the expansion of the Roman empire, the Romans were able to get all the way to Britain, which is farther west than the Scandinavian countries where these myths originated.
The Vikings made many expeditions into Britain. So it is a very realistic thought that the Vikings could have been exposed to the stories of the Greek and Roman gods. It is also possible that the Vikings could have extrapolated parts of the Roman stories into their own. The only two existing primary sources of Norse mythology are the Prose (Elder) Edda, and the Poetic (Younger) Edda. These were written about one thousand to eight hundred years ago respectively.
The second factor has to do with mythology as an extension of the society that fashions it. I see mythology as an attempt by a people to explain the powerful forces which affect and shape it, that are beyond its control, such as weather, the elements, and nature. I also see gods as being characters that have many similarities with the people within the society. The gods and goddesses are powerful beings capable of super-human powers, but nevertheless are characters fraught with very human frailties and flaws.
In this way they created gods that had similarities with the common man in the society. This made the gods more tangible and easier to identify with. I think that this was necessary because they were not yet at the societal maturity level to have a single god on a cosmic plane. Most of the comparisons will be examining the similarities between major Greek and Norse gods, as well as creatures, stories and specific symbols which are featured in the mythologies.