The best parts of a startup include having a great team, building lifelong relationships, connecting with other CEOs, celebrating victories, always having something important to do, being challenged, fulfilling the desire to build, not working for 'The Man', and staying young at heart and curious.
The toughest parts of a startup include being let down by co-founders or partners, making big mistakes, facing challenges when unable to meet goals, and the perpetual difficulty of recruiting.