Surprise!

The ending of The Scarlet Letter was unexpected, but only partially. By now, we've been conditioned to expect plot twists and surprise endings. What half-decent novel or film doesn't have them? From Ender's Game to The Dark Knight Rises, the unexpected has started to become, if you'll excuse the cliché, the expected.

[Spoiler alert]

Near the end of The Scarlet Letter, Hester and Dimmesdale make plans to escape Boston and leave behind their sins. To more cynical readers, this seems too good to be true--a fairy-tale ending, if you will. This sentiment is manifested in Pearl, who foreshadows the tragic ending when she makes Hester put her scarlet letter back on. In addition, Hawthorne's reputation as a "brooding" Romantic also does not bode well for Hester and Dimmesdale. Despite all this, escaping with Hester is still the most rational choice for Dimmesdale; it's most likely what we would do if we were in his position. So, in effect, we're expecting the plot twist even though it's not the most reasonable thing to do.

So why does Dimmesdale do it? Why does he confess to the world, effectively committing suicide and deserting Hester and Pearl, when he has a perfectly good escape plan?

Earlier, I called the ending tragic. However, from Dimmesdale's point of view, it's not necessarily so. His confession is his catharsis, a cleansing of his soul from years of accumulated guilt. He's finally at peace with himself and God and ready to receive his judgement. This illustrates how painful secrets can be, and the sheer relief that comes with their revelation. You could also argue that Dimmesdale was just a coward who couldn't just get on with his life, but that's a matter of opinion.

In the end, Dimmesdale is very similar to John Proctor. They both valued the truth over their own lives.