Why is the US government spying on Americans?

The US government with assistance from major telecommunications and Internet companies has engaged in massive spying programs on millions of Americans. US President Barack Obama has defended the top-secret programs saying “they help us prevent terrorist attacks” despite their “modest encroachments on privacy.” Scope of phone and Internet surveillance has caused alarm among privacy advocates. The man behind the leaks, Edward Snowden, an ex-CIA employee working as a contractor at the US National Security Agency, said he leaked documents and details of a top secret US surveillance because he felt the United States was building an unaccountable and secret espionage machine that spied on every American.

Why is the US government spying on Americans?
What do they say about the intrusion of privacy rights of Americans?