As you grow and mature throughout your life, you are preparing to experience the possibilities present in your birth chart. As time passes, the planets begin to pass over the positions they once held at the time of your birth. As each planet transits these points, energies coalese to activate that particular planet's sphere of influence in your life. It's at these times that you're most likely to experience change in the areas of your personality affected by that particular planet. This is why you'll strongly experience the energies of certain planets at certain times in your life, but then those same energies will seem to fade away.
Opportunities arise based on the nature of the transiting planet.
Thus, they usually deal with the simple, more mundane areas of our lives. Hence, the effects of the transit of one of these planets can be felt for years, thus making the experience much more profound.