Happenstance can be a powerful source in a story. A well placed coincidence can provide that important opportunity to move the story forward. Just make sure happenstance doesn’t drive your story, or it’ll feel contrived.
What drives the story is the characters. They must weigh the options and make choices. They can’t just be swept along by circumstances. This makes for wimpy characters and a boring story. That’s not to say your hero must dominate the plot. But he must make a difference. He must take the initiative in the face of opportunity, not respond to circumstances beyond his control.
The difference is noticeable. What if Zorro just waited for the situation to resolve itself, like the townsfolk are prone to? That wouldn’t be interesting at all. What if Manny Calavera just decided to do his job, not to shake up the system? He’d still be rotting in the Department of Death. What if Luke never asked out Lorelai?
So happenstance can provide the opportunity, and then only occasionally, or else it’ll seem like the writer is making it too easy for his characters. In any case, happenstance can’t provide the initiative. A great story lets the character shine by taking control over the situation and giving us someone to root for.