IMHO, for success, the cofounders have to work as a team, which can only be achieved if there is friendship. You can either have this friendship relationship before you start (you start a venture with a friend who becomes a cofounder), or you can try to develop it later on (you find a cofounder who later on becomes a friend). The problem with the friendship-later model is that with these matching websites, there is no guarantee that you will get along with the co-founder to the extent that you develop a friendship relationship.
Besides respect, friendship gives you trust and commitment. Ideas of venture do change along the way. A friend is committed to you before the idea of a venture came along, hence, he will be flexible with the change in the idea, the company, the profits, etc. On the other hand, a co-founder (who was not your friend) joined you based on the idea (or the money), hence, he will be less flexible when things change (if no friendship develops along the way).
Besides respect, friendship gives you trust and commitment. Ideas of venture do change along the way. A friend is committed to you before the idea of a venture came along, hence, he will be flexible with the change in the idea, the company, the profits, etc. On the other hand, a co-founder (who was not your friend) joined you based on the idea (or the money), hence, he will be less flexible when things change (if no friendship develops along the way).