Serendipity is making an unexpected fortunate discovery, while you're pursuing something else.
Human beings need to plan, to think long range, to pursue goals and a hierachy of values. But life is full of surprises... many of them great, but some annoying, frustrating, absurd, or disastrous. Adapting to circumstances as they change, always learning, facing even the negative with fortitude and a smile, means even bad news can be a good opportunity.
For an inspirational illustration, see Cyrano De Bergerac .