Most of us live life by default. We work through our daily routine, mostly unaware of actually doing anything. We may, at the end of the day, say "where has the day gone?" Eventually this will turn into "where has the year gone?" This occurs because we live life by default. We understand what we need to do and no longer need to apply our awareness to the tasks.
We wonder where the time went because we are unaware of what we are doing while it passes. By aware, I mean stepping outside of your own sense of self and seeing what is happening as if you are an observer to your life. Like a member of the cast in a play, it is difficult to see the whole plot, but as an audience member, it becomes more clear.
Being aware is not something that must be achieved every moment of the day. Awareness should be sought at least once a day though - it brings a sense of calm. To be aware, start by doing this simple exercise. when you get to the end of this paragraph, stop reading. Once you have stopped, take two deep breaths and tell your self to relax, let your tightened muscles go limp. Now, ask yourself what you are doing. Notice the screen you're reading this from. Then look around you, what do you see? What objects or people are in your surroundings? Look at as many details as possible; color, texture, identify other attributes of your surroundings and mentally name them. Also, notice your sense of touch; feel your body pressing down on the chair. Now notice temperature, is your skin warm or cold? What sounds do you hear, how many different sounds can you identify? Do you smell anything? What does it smell like where you are? Be aware of your surroundings for a minute or two.
The regular practice of awareness is called meditation and can be very beneficial. For more information about meditation see the links below.
Meditation is often associated with religious activities, but it is not necessary to be religious to meditate. The act of awareness in itself is beneficial.