Of all the words of mice and men,
The saddest are: "It might have been"
Kurt Vonnegut
And it's true. We should strive to take every opportunity, because those we miss, we spend a lifetime wondering what could have-would have happened.
Painting beautiful futures that can never be, because we didn't dare to try. What might have been is a world of infinite futures and possibilities, all of them lost.
So take every chance, say hello to that stranger, agree to that trip, say yes, don't hesitate and let the moment be lost. And make as few of 'might have been' moments as you possibly can.