For historical reasons, relations are easier between the Lib Dems and the Conservatives than between the Lib Dems and Labour. Psychological theory can help to explain why.
Transactional Analysis categorises each participant in a discussion as a Parent, Adult or Child. Most productive conversations or negotiations require both parties to operate on an Adult-to-Adult basis.
Political parties have to accept each other as equal Adults in order to develop productive relationships. In general, the Liberals/Lib Dems and the Conservatives have managed to do this most of the time. The Conservatives and the Liberals have a long history (over 150 years) of dealing with each other as worthy opponents.
However, the relationship between the Liberals and the Labour Party is more problematic. Between 1903 and the First World War, the Liberal Party essentially acted as a nurturing Parent to the Labour Party, through the Gladstone-MacDonald Pact which helped Labour achieve a foothold in Parliament. Very quickly after the First World War the Labour Party overtook the squabbling Liberals and assumed a dominant position in Parliament.
So, between 1914 and 1922, positions reversed from the Liberals as the Parent and Labour as the Child to the opposite dynamic. This is not an easy background from which to negotiate. There have been brief interludes when the Liberals/Lib Dems and Labour have treated each other as equal Adults (the Grimond and Gaitskell era and Ashdown and Blair could be considered to be examples).
In the aftermath of the 2010 election, Gordon Brown essentially took the position of a strict Parent treating Nick Clegg as a Child, while in many ways Nick Clegg attempted to do exactly the same to Gordon Brown, telling Brown that he would have to step aside as a pre-condition for any LibDem/Labour coalition.
Labour and the Lib Dems need to move beyond this unproductive relationship, especially if the parties may need to work together in a future coalition. If this happens, then there will be enough day to day discussion for the parties to begin to forget about the past. But, in the meantime the parties have to get into the habbit of treating each other as Adults.
Viewed through the prism of Transactional Analysis, a friendly Adult discussion between Ed Balls and Nick Clegg could be seen as a significant breakthrough.