JK Rowling has recently commented that she persisted with the romance between Hermione and Ron, despite this being less likely than Hermione and Harry, reported variously in the press, including here in the Guardian.
There is a third way, a path that mirrors the hard truths of modern life in the Harry, Hermione, Ron love triangle. It was rational for Hermione to choose Ron in the aftermath of madness, and the wizarding world needing reconstruction. He was safe, uncomplicated, undemanding.
But within a few years the mundanity of life with this very ordinary person would grate on someone with her extraordinary gifts. He would not “get” her, or understand her need to be challenged, intellectually, magically, perhaps sexually.
As for Harry, well Ginny was always the rebound girl. After crashing and burning with Cho, and being unable to match the martyr status of Cedric Diggory with his own heroics he fell back on his own safe option, the kid who had a crush on him from the start. Whatever he felt for Hermione is subsumed in that overwhelming desire to be noble; orphans are never able to believe they deserve at the expense of others.
But roll them forward a few years, when Ginny’s clinging neediness and increasingly frequent mental health issues from her brush with Tom Riddle will be tearing the fabric of the relationship. Harry can’t abandon her, it would be ignoble, but nor is she the anchor his own instability needs.
It leaves Hermione unsatisfied, and Harry trapped. Their seeking solace in one another is inevitable.
I have written some fan fiction elaborating on this theory – take a look