Why is US afraid of North Korea?

The United States isn't necessarily "afraid" of North Korea.

North Korea considers itself to still technically be at war with the US even though the Korean War ended 60 years ago. North Korea still has a massive military (their army is one of the largest in the world), and has access to nuclear weapons. Additionally, they are closely aligned with China, a country rivaling the US in sheer military and economic power. However, North Korea has no way of really directly attacking the US on their own; they only have a small navy, and their air force is no significant threat to the US's massive air forces. Rocket tests have shown that North Korea is probably not capable of hitting the US mainland with nuclear or chemical weapons.

North Korea is, however, a threat to South Korea and Japan, who are close allies of the US; since the Korean War ended, North Korea has made it clear that it intends to conquer South Korea someday. Hatred of the South and the US is one of the ways the North's government keeps their people under control- they focus hatred towards those countries, so the people forget that their life in the North isn't actually very good. In keeping with this theme, the North often threatens to attack the South, Japan or the US and sometimes does actually attack the South (for example, in November 2010 they launched an artillery strike against a small island off the coast of South Korea and killed/wounded a few people).

So it's not so much that US is "afraid" of North Korea, it's that North Korea is sometimes erratic, often throwing out threats of extreme violence for the smallest reason- or no reason at all.

Answer 2The United States has concerns against North Korea mainly for two reasons:

1. North Korea has the atomic bomb and the means of conveying it to the US
2. North Korea threatens the US allies in the region; mainly Japan and South Korea.