Do not agree with Paul Graham on finding cofounder
I don't 100% agree with PG, I believe friendship can also be built via start-up. Besides, how, or how many years you know each other doesn't necessarily mean how the chemistry is.
If someone in my team is not aspired by our team goal, he should leave; if someone doesn't put his heart on the project, i will ask him to leave the project no matter how many years we have known each other.
I think a major responsibility of a core founder is to manage the team morale and make sure everybody in the team consistently love the team goal. if there is a point that everybody feels doomed to fail, it's the time to reset the direction and recharge the team. The core founder shall have the passion, persistence and ability to drive the team forward.
Friendship drives entrepreneurship and vice versa.