How did Kevin Systrom find his Instagram co-founder Mike?
Kevin's first investor gave him money and instructed Kevin to find a co-founder because in his mind, the best companies in the world all had co-founders. A team was just more likely to succeed than a solo founder.
Kevin had met Mike Krieger before and was impressed with him, so next time they met, Kevin inevitably pitched the idea.
They met up for a day and prototype-worked together on a fake project. That was an enjoyable experience. Eventually, the two partnered up.