why do we have a coalition government

The current government we have is called a coalition government because it is made up two political parties. After the end of the last election no party had a large enough majority of MPs to form a government that would work, this either meant that they had to run the election all over again, or some people had to decide to work together.

The Liberal Democrats (Lib Dems) - the third biggest party in numbers of MPs, and the party in the middle of the biggest two (Labour and Conservative) in terms of their beliefs - went into talks with both in order to decide if they wanted to make a deal with either.

In the end they decided they would support the Conservative MPs - this means that on important issues the Conservatives could count on the Lib Dems votes and parliament could function (pass bills and policies).

There was a coalition agreement where the Lib Dems were able to get the Conservatives to take on a few of their policies, and to pick issues which they didn t want to vote with the Conservatives on, so they get something out of it, too.
According to some reports, he will step aside to allow Finance Minister Evangelos Venizelos to form a coalition government.

It could be a part of some coalition government in the future. This means that if he wants a majority in Parliament, he will have to form a coalition government.

McCain dismissed Western concerns over an Islamist-led coalition government, if there is one, when asked by The Daily Beast. That will be a difficult balancing act, because his social-democratic party PvdA is in a coalition government with the VVD.