I think the US puts a much higher value on personal freedoms than other developed countries do and this is particularly the case in gun ownership. Western European countries typically have accepted restrictions on weapons to benefit society as a whole. US society has accepted much higher death rates from firearms* as a trade off for the right to bear arms. It seems strange viewed from Europe but Americans are not just Brits with a different accent.
* Number of Homicides by Firearm 2007 (UN statistics, as reported in the UK's Guardian newspaper)
England and Wales 41
France 35
Denmark 15
US 9,146