Coalitions are often formed in situations where no political party emerges from an election with 50% +1 of all the seats. Multiple parties will form a temporary alliance to form a government. This is a coalition. As a result, none of the parties joining together get to implement their manifestos in their entirety. Instead, the parties must find common ground, put across their red lines and compromise where necessary. In practice, this leads to a cabinet made up of ministers from all parties involved and the parties voting together on most key issues. Under the UK's first-past-the-post voting system coalitions are unlikely as the system tends to produce majority governments. The 2010-2015 coalition government was an exception, formed when the Conservatives fell short of a majority. Coalitions are much more common in the devolved administrations of the UK, which use voting systems that tend not to lead to majority governments. The first two Scottish parliamentary elections resulted in Labour-Liberal Democrat coalitions and Wales has had a Labour-Plaid Cymru coalition. Both now have minority administrations, in which the SNP and Labour respectively are in government but rely on other parties for key support.
Coalitions are also much more common in Europe due to more proportional voting systems. Germany's Angela Merkel currently leads a 'grand coalition' of the country's two largest parties, and Finland has a coalition government made up of three parties. The Danish political drama 'Borgen' shot to fame at home and abroad, depicting the complex nature of coalition life.
One argument in favour of coalitions is that they lead to broader governments, allowing more of the electorate’s voice to get a say in terms of policy. This is the case as policies do not just come from one party. Coalitions water down the extremes of different parties and end the 'tyranny of the minority'. For example, the current Conservative government may have won more than 50% of the seats in 2015, allowing them to pass almost anything they want, but they only achieved 37% of the vote. Proponents of coalitions argue that coalition governments lead to parties to working together as they help create unity, as well as weaken the nastiness and polarisation in politics.
One of the most common reasons coalitions are opposed is the argument that 'no one voted for a coalition'. Individuals vote for parties, not for a policy blend of different parties.
Following on from this, one other criticism is that the main agreements in coalition formation are often agreed in backroom deals. The electorate might end up voting for a hung parliament, but they do not get a say in how parties operate with one another.
Opponents of coalition government argue that it is unfair that in some cases a coalition government can exclude the party that 'wins' the election - the largest party. One party may get the most seats, but a combination of other parties willing to work together could form a majority and block the ‘winner’ from taking power. This has yet to happen in modern UK politics but if the arithmetic adds up then it could be possible.