Replies to these kinds of questions are difficult.....and obvious at the same time. Human motivations often center around base instincts for wealth, power or sexual gratification.
More or better weapons can aid in the accumulation of any of those facets. This goes on from the scale of one-on-one to country-to-country. Not to mention that some economies are practically based on supplying weapons to other states - thus they have little incentive to limit production or availability.
But sadly, this is hardly a new phenemenon.
"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin