It's easy, right? Taking on more responsibility and becoming a manager at your company is a piece of cake. You've worked there for years and know the drill, so how hard can it be?
It's probably way more challenging than you imagined. Going from being just another person on a team to a manager is a big step, not only in name and function, but also in behavior and difficulty level. While before you may have been only managing your own work and projects here and there, now you are a manager of staff members and their work. You may now find yourself supervising happy-hour buddies with whom you previously complained about a manager. (Hey, that's you now!)