The main character(s) of your story should at least try to solve their own problems. It’s okay if they get help, but if they simply languish in endless misery until some third party hero comes along to save the day, then my question is: Why isn’t the third party hero the main character?
You probably know this, but I’m going to go way out on a line by suggesting that the rule still applies if you are writing in the *romance* genre. This is where I see the most difficulty following this rule, even in published fiction. The hero and heroine simply cannot be together because of this and that, but fear not, fate will step in and intervene! Queue matchmaker. (But not the sound, please. 🙂 )
In the best romance, one or both of the main characters are striving for happiness. They may honestly believe they are happier apart, or that the other is better without him/her. But they’re trying. Giving in to misery, accepting things the way they are, even for noble or religious ideals (ie I can’t possibly get a divorce), makes the character pitiable, not sympathetic. (Note: This is why I almost always avoid Historical Romance in which the hero or heroine is already married to someone who has to go — fate has no choice but to step in and make a widow(er).)