I saw this earlier and this is why I think a co-founder should be a friend.

'What is a friend? I will tell you...it is someone with whom you dare to be yourself."

This is so important when working together, you need to be able to tell them exactly how you feel, to receive honest feedback and take it in the right way.

Brothers/Sisters and or friends, I think, make the best co-founders. As it is with these people that we can be ourselves which can only be good for your startup. If you go into business with someone then you need to trust them implicitly, if you have this level of trust in place then I firmly believe your partnership can flourish and therefore so can your startup.