Since it usually means the party in government has a) a majority of seats and b) enough of the popular vote to justify being in government. See the USA, a two-party system which effectively has weighted FPTP in the electoral college.

You also need to have some areas that support party 1 and some that support party 2, with a few marginal constituencies which switch allegiances every so often. Otherwise you get situations like what happened in Lesotho in 1998, where one of the parties got 24.5% of the vote but only 1 seat out of 80 – this started a mini civil war which ended in a form of mixed PR being introduced.

The problems arise when party number three starts being taken seriously. You're seeing that happen now – you're probably used to it in countries where they let lots of little parties in, but it's a bit of a novelty for us.

Now, as a classroom exercise, design an electoral system where seats are allocated fairly according to how many votes each party gets, but where extremist parties like the BNP find it very hard to get seats, and where it's actually possible for someone to form a government without ending up with a coalition that nobody voted for. Answers on a postcard to Nick Clegg, who'll give a knighthood to whoever can find a solution.