Why do you think they fought so bravely? Because warriors who died bravely in battle were guaranteed a place feasting at the high table in Valhalla!
Incidentally, there's a tale that at one drunken feast, the question was mooted on how long a man lived after his head was cut off. One Viking volunteered to demonstrate by continuing to speak for as long as he could. However, not a squeak came from him, nor from the next volunteer. Then a more "intelligent" Viking pointed out that without a throat, of course one couldn't speak, but he would start blinking his eyes and continue to do so after being beheaded.
Finally, the conclusion was reached that death from beheading was instantaneous, and they all drank a toast to the three new warriors feasting in Valhalla.
PS. I'm of Norwegian Viking stock, but I try to keep my head!