1. PG specifically writes this in his Startup FAQ (www.paulgraham.com/startupfaq.html)
"Most successful startups have more than one founder, and usually the founders seem to have been friends for at least a year before starting the company."
2. Yes.
2. Yes.
There are lots of reasons not to found with friends. Often, it's just that you're choosing someone you'd have fun working with over someone who's the right fit. Other times, your judgment is impaired by your affection for that person, and you can't determine whether they're a good fit or not.