How do you keep yourself motivated, obviously you have been very successful, and have built great companies. Is it the mission behind the companies that keeps y

How do you keep yourself motivated, obviously you have been very successful, and have built great companies. Is it the mission behind the companies that keeps you motivated? Or is there someone or something? I am sure the daily drills of running billion dollar companies can be tiresome but what is your secret for being so motivated and what drives you now to keep pushing the limits.