Of course they do. Sure, co-founders can fight over product design, what type of pizza to order for lunch, or which programming language to use, etc. But where

Of course they do. Sure, co-founders can fight over product design, what type of pizza to order for lunch, or which programming language to use, etc. But where I've seen most startups collapse and fail is over contentious issues such as splitting ownership and compensation. The inability to come to an agreement on this is when relationships turn sour and co-founders split. It's rare to hear about someone leaving a startup because s/he disagreed with the startup's product vision. Don't treat this issue lightly and get a lawyer if necessary to write up a 1 page agreement between the both of you. Things can get ugly.