The ever-present danger of mixing business with pleasure (including friendship). Mark has always been the kind of guy who only dances to his own tune (ha!), except if you are Chet Atkins or Bob Dylan. I certainly think Jack knew and accepted that. Mark was the star, and they would never be on equal terms artistically. However, because of their friendship, Mark should have showed Jack the respect and decency to at least offer some kind of explanation. If Mark no longer felt a desire to go on with a project involving Jack, then they should have had that conversation.