Because the UK had a stable two-party system for so long, it has become difficult to contemplate a future in which coalition government is normal. Yet a wider historical perspective would show that there have been frequent times when the two main parties splintered and deals had to be struck to keep a majority afloat or a third party managed to prevent one of the others from gaining a majority; going further back, the very concept of a party was unknown back in the 17c and 18c when faction and personal interest dominated and political alliances were formed as much from individual likes and dislikes as from a broad agreement on policy.
We have a parliamentary system. The vote this Thursday will allow us to choose our representatives. We do not get a vote on who is to become Prime Minister, nor on which parties may form the next government. No party is entitled to claim a right to form the government unless they can show they have a working majority in the Commons. Any group of parties that can do so are entitled to vote down any government lacking such support and to claim the right to form the government themselves. For parties to disdain the idea of coalition is a form of contempt for democracy. If most of us vote for several parties who are able to work together then this is a very good expression of democracy.
Furthermore, although politicians will say that coalitions inhibit them and they dislike the horse-trading, as much of this goes on within the parties as between them. Both Labour and Conservatives have had searing internal rows on divisive issues, including our future in the EU, trade union rights,immigration, privitisation and others. There are special interest groups within the parties that have to be appeased, sometimes going against mainstream policy, although one could argue that with party membership way lower than it used to be, the parties are less broadly representative. But coalitions within the parties produce a broad consensus that makes them appeal to a wide range of voters and the same principle should apply between the parties.
In short, there is no reason why my political interests should be the same as yours, but in most cases it makes sense for us to work together to secure the most reasonable result.