Why was there just a federal election?

Why would PM Stephen Harper order a federal election, when he is already Prime Minister? It is most likely because prior to this election, the Conservative government under Harper was a Minority Government. A Minority Government is a government that has less than half the number of total seats in parliament. I think Harper's goal was to gain more seats as to become a Majority Government. The Prime Minister and Cabinet in a Majority Government have much more control and can enact whatever policy they want. To obtain a Majority Government, he would have to have won 154 or more seats, but Harper and the Conservative Party won 143, 11 less than they needed.

Here's a video on his decision, showing his announcement.