Recently in Canada, there's been a debate by progressives in society and within the two centre-left political parties about how to ensure victory in the 2015 election, or at least to prevent anymore destruction from Prime Minister Harper and the Conservatives. One of the main ideas being put forward is the idea of a merger between the Liberals and New Democrats. Polls indicate that there remains majority support for such a merger among members of the two parties. In spite of this, a merger between the two parties is very unlikely.
Merger is a bit more confusing than it really has to be, but a lot of that has to do with the history of the parties, and the culture that's been created within each party based on their funding and membership strength.
Historically, the Liberal Party was generously funded by Bay Street, similar to how Wall Street has recently been more willing to fund the Democrats down south. Obviously, the NDP is significantly more funded by unions, and continues to be funded through unions and small donors even after the campaign finance laws put forward by Chretien. Any merged party would instantly have difficulties reconciling unions and corporate executives, both of which would demand dominance in the new party.
Beyond culture, there's ideological differences that have to be taken into account for. Obviously due to the influence of corporate money in the Liberal Party, they've always been more deferential to corporate power and capitalism in general than the NDP has been. Even in 2005 the Liberals were promoting corporate tax cuts as a means of stimulating the economy and frittering away a budget surplus in the billions. Their other economic failure is to continue promoting free trade without any kind of barriers or protections for Canadians. It's a point of pride among most NDPers that we vote against trade agreements that ship jobs overseas, whereas many Liberals see those trade deals just as importantly as many Conservatives.
Liberals have also been far more willing to use force, including in Afghanistan and far beyond the original terms of the agreement put in place by NATO. Opposition to Iraq was nice, but that was based on prevailing public sentiment and not on any real opposition to the war.
I'm not sure if this would count as a 'real' reason to oppose merger, but it's been my experience and observation that there's an attitude difference between the two parties. The Liberals, in spite of repeated defeats at the hands of Harper and the near destruction of their party west of Thunder Bay, continue to have an arrogance about them that seems out of place on the third place party. There's an insistence that next election they'll be returned to power, based simply on the fact that they were the 'natural governing party' for a century prior to being rejected.