Someone pointed this out about themselves. Because of the flood in Halo CE, 2 and 3, they must always have a shotgun on them whenever possible no matter what game their playing. I just realized I do the same thing, and without a doubt this is the reason why. In every game I make sure I hold onto a shotgun because that's exactly what I always did in Halo to fight the flood. And I bet all you Halo players do exactly the same.