conspiracy is a thing for a reason

Multiple people working together are fundamentally more powerful than a single person working alone. Governments recognize this fact, which is why conspiracy is a crime: several people plotting together are significantly more dangerous than a single criminal. Similarly, YC recommends that startups be founded by at least two people.

Why is collaboration so important?

• Motivation: it's impossible to maintain direct interest in any single problem for very long, especially if you're working alone. Humans are social beings and our motivations are heavily tilted towards fitting in with the people around us and making them happy. This ordinarily makes it hard to care about a problem that no one around you cares about. Working with a collaborator turns this on its head: making progress is a core part of the collaborative relationship, so you'll want to do this for social reasons even when other motivation is lacking. At the same time, you're also providing them with social motivation, creating a positive feedback loop.
• Grounding: A collaborator forces you to come up with ideas and plans that make sense. They will inherently have a different perspective than you---even if there are broad similarities, they will be at least as different as you are from you-next-week, which is already significant---and can push back on poorly-thought-out ideas.
• Stable abstraction: The mind has very high internal bitrate; communicating with another human has very low bitrate. The only way to communicate effectively is to abstract: to agree on words that stand in for otherwise complicated concepts or explanations. When I work by myself, abstraction is painful: I can see in my mind the full complexity of whatever aspect of the problem I'm looking at, and I have a hard time throwing most of it away. But with a collaborator, the low-bitrate requirement forces abstraction, which helps develop a new conceptual structure for thinking about your problem in more powerful ways. communication is processing
• Practice articulating ideas: related to, but separate from stable abstraction. Even in an area where the abstractions are fixed and well-known (elementary math, perhaps), it is one thing to recognize those abstractions, but quite another to train yourself to be able to speak fluently about them. Working with a partner applies articulation regularization, forcing you to focus on ideas that you can coherently express. This is an instance of generative vs discriminative modeling: having a partner forces you to practice generating statements about your topic, not just evaluating them.
• Ambition