More than anything else it seems to be the near-total lack of exposure, when living in larger cities, in contrast to those living in small towns or rural areas.
Bingo! People who grew up in a city and have never fired a gun don't understand the pleasure that can come from shooting with friends or hunting with family. They don't understand the use of a firearm as a tool. Likewise, people that have grown up in a rural area don't understand the level of gun violence in major metro areas and can't understand why someone would like to restrict access to firearms. Sometime those worlds can collide and common ground in found.