The moment your heart is broken, you feel like an open wound. You want to patch it up, cover it with something. When a person I know breaks up with their significant other, I can sympathize why they suddenly look for someone else. It's a way of coping, of making the pain hurt less. But it's also not what I would condone.
During this fragile mental state, your senses are clouded. It's akin to picking up the next guy or gal or you meet in a bar when you're drunk. Sure, all's well at the start--you think that you've finally found someone, although in some instances a part of you knows that this isn't the "one," someone you simply happened to settle for. And then you slowly become sober and the shortcomings of this new person become more and more apparent. Perhaps you were too hasty in committing. Or maybe you meant the words that you said to him or her, but it seems so far away now.
I've been to that place and I know that's the point in time when I need to be the most skeptical. It helps that I'm unappealing and no one is usually intested in me.