Opportunity can be defined as a chance particularly given to someone as a means of favour.
Whenever a door of happiness closes another one would definately open but mist often we tend to keep looking at the closed door and waiting for it to open instead of us moving forward and looking for the next door. Any door which closes means its time to move higher, dont sit and wait for the closed to to be opened, move forward and see the new open door.
These are the main reasons why opportunities are missed, waiting instead of moving forward. Many times opportunity knocks, but before you unlock the door, it might have been late because you were busy waiting for a closed door to be opened.
Opportunity is a product of grace, mercy and favour, never fail to take advantage of the grace you have.
We know the story of Esau in the bible who had the opportunity but missed it because of hunger, he sold his birthright. He was saddened, wept bitterly but it was too late and the opportunity was gone.
Use whatever opportunity you have today. Let grace speak for you.