Joint ventures (as opposed to partnering with someone to set up a business) generally well when two criteria are present:
1. the two entities jv'ing together have a strong reason to do so (e.g., a better value proposition together, complementary capabilities, complementary assets (e.g., one has a great technology, another has great relationships)
2. Their motivation for staying together is stronger than for moving apart (and the jv agreements are structured in such a way that everyone wins by staying together and everyone loses from separating the JV)
But the first thing is to always always always understand the strategic and tactical motivations for doing the jv from both sides