How to create a strong thesis statement for "Happy Endings"?
In order to write a compelling thesis statement, you must develop a position on some arguable point in regard to the text. In part F of "Happy Endings," Atwood (or her narrator) concludes by stating, in part, that
You'll have to face it, the endings are the same no matter how you slice it. Don't be deluded by any other endings, they're all fake, either deliberately fake, with malicious intent to deceive, or just motivated by excessive optimism if not by downright sentimentality. The only authentic ending is the one provided here: John and Mary die. John and Mary die. John and Mary die.
What Atwood writes in this text is called metafiction, meaning that she's written fiction that takes, as its subject, the standards and "rules" of fiction. And she's taken a pretty strict position on what good fiction does. For her, good fiction (ironically) tells something that sounds a lot like truth (or nonfiction); good fiction must, on some level, tell a truth about life, and the only absolute truth about life is that it ends. This is certainly an idea with which you can agree or disagree, and so, you might consider constructing a thesis that takes a position on this statement.
Further, Atwood argues that plots are just "a what and a what and a what," and she encourages us to think more deeply about "How and Why" things happen rather than "what" happens. What do you think she means by this? One alternative to my suggestion above is that you offer, by way of a thesis, an interpretation of what Atwood means here. You could even offer, as your thesis, an interpretation of the quotation I provided above as well. These ideas both seem to encapsulate the meaning of "Happy Endings," and so they'd be great places to start to formulate your own ideas about what the text is saying and if they are ideas that you support.
In order to answer your question more effectively, I'd have to know what type of essay you are required to write. There are many different kinds: Compare/Contrast, Argumentative, Persuasive, Expository, etc. But for a generic answer I would have to go with telling you that your thesis statement should have words such as "is", "should" or "must" in it. These words are strong enough to support statement about what you really think about the topic (although remember that saying "I think" isn't usually what you need to write.)
The United States should rethink the illigal immigration issue.
The European Union must reconsider Greece's economic standing.
The welfare system in America is costing citizens billions of tax dollars each year.