Bruce wanted to marry Selina, but unfortunately, she loved Batman.
It sounded like a bizarre reason. Wasn't Bruce Batman? Yet Joker wasn't the only one who knew the truth.
Bruce Wayne, Gotham's greatest playboy, truly wasn't Batman. Though the two might have shared the same body, they were completely different people.
Batman was the one who truly existed, while Bruce Wayne had actually died alongside his parents that night in Crime Alley decades ago.
Besides Joker, plenty of people knew this. It had practically become Gotham's open secret.
Perhaps it had been someone else's scheme, or perhaps she simply hadn't thought it through, but the result had already happened: before her wedding to Bruce, Selina left behind a letter and vanished without a trace.
She had thought she was ready. Lois had even taken her to the Fortress of Solitude for a bachelorette party, where they sampled all the alien liquor Clark had saved up and watched the Superman robots perform a steamy striptease.
But after returning to Gotham, Joker found her.
"Gotham cannot be without Batman."
Catwoman had originally intended to kill him, but what Joker said next made a great deal of sense:
"Only Batman is real, while you're merely marrying a fake named Bruce—a void, like an empty shell without a soul."
While Selina sank into thought, Joker had already slipped away.
Afterward, the bewildered Catwoman went to discuss it with her girlfriends. One after another, they urged her not to marry Bruce, because while the body might be the same, it was not the soul that met her atop skyscrapers at midnight.
She would never be happy that way. Batman trusted no one, especially Bruce.
Anyone who knew Selina's true identity had to be someone she trusted. In the end, Catwoman was persuaded.
So she left