In politics, 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, in part due to their history. 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). However, 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, having to tell Brown that he would have to step aside, as a pre-condition for any LibDem/Labour coalition. (Incidentally, it is debateable whether Brown’s departure was really just a LibDem pre-condition, or if it was also a Labour pre-condition.)
Viewed through the prism of Transactional Analysis, it is no surprise that the LibDem-Labour coalition negotiations failed, but the Conservatives and the LibDems managed to strike a business-like agreement.