In truth it's all a matter of perspective. A developer tries to look at everything from multiple perspectives and accounts for each accordingly. The idea is to establish the game so the gamer follows a certain mindset regardless of their backgrounds. That is to say anybody should be able to pick up the game and play relatively quickly on a short learning curve to at least function to the point they can make progress in the game without particular difficulty. Then it's a matter of coaxing people into thinking a certain way so they look for certain things or recognize certain patterns. Eventually the goal is so that every player will instinctively know where to go next and desire to go there. Though because this can't be completely accounted for we've developed a system of telling the player explicitly where they need to go in order to progress.
There's a basic principal to be found in almost every successful book, movie, and game. There is a challenge or conflict to be solved or resolved and a series of events that relate to the final goal. There's a set amount of resources available to the primary interest usually a character which can be used to pass through each event and obstacle using the skills and knowledge presented previously. In video games most of this information is presented with an on screen display commonly referred to as the Heads Up Display or HUD while other information and assets are tucked away in a menu system for things like inventory and configurable game settings. These systems are designed to be esthetically pleasing and highly intuitive such that any viewer can guess with relative accuracy what something on the screen means or represents.
Each different perspective is like a different puzzle with different pieces to put together. When you're finished sometimes you have extra pieces other times you have to take pieces from a different puzzle to complete it. Then once each puzzle is completed you now have your assets to put the game together. This means each completed puzzle is now but a piece in a much larger puzzle to put together so that everything fits perfectly which often requires going back to old puzzles and making new pieces for them or doing something different with the ones you already have.
Your motivation comes from all sources, some more than others just the same as your inspiration. Though just as often the sources of your frustration comes from the same sources. It's addictive to say the least. You become engrossed in a world you're creating. Often so completely that you forget to eat or sleep, you become almost detached from your body as you stand upon the precipice of greatness. When you're doing it right it's like the world has a mind of its own, willing itself into existence through you and all you're doing is revealing the sculpture hidden in the clay. When it's done you'll look like death itself but feel like a god of your own universe.