Everyone has opportunities and no opportunity lasts forever. So, everyone has a revolving door of opportunities that come and go. But, people tend to forget, which is why people tend to take opportunities for granted, thinking every opportunity will last forever.
Sometimes you are the opportunity for someone else. Accordingly, people will take you for granted and think that your time is limitless. When they do, don't stick around for them. It's always tempting to stick around while ambivalent people ponder nothing infinitely; don't.
One of the best opportunity teachers is the opportunity missed. Never miss your teachable opportunity to teach.