Readers complain that my characters explain too much in their dialogue. What am I doing wrong?

It sounds like you may be using expository dialogue, which stuffs information into the character’s speech in order to convey it to the reader. Expository dialogue sounds like this:

“Yesterday, I walked by the street corner where you and I first met on that rainy night when we fought over a cab. Good thing I let you have it or we wouldn’t have gotten married two years later.”

Of course, the listener—the man’s wife—already knows about the first night that they met. She doesn’t need to be told that it was rainy or that they fought over a cab and she certainly doesn’t need to be reminded that they married two years later. It’s still probably vivid in her memory and if it’s not, just one detail might be enough to bring it back. A more realistic line of dialogue might sound like this:

“I was at Bleecker and Sullivan yesterday.”

Can’t you just see the smile spread across the woman’s face? Mention of that intersection could be enough to jog her memory about the rain, the cab and his delayed chivalry. Of course, the problem with this line of dialogue is that while the couple knows the details, the reader doesn’t yet. You can remedy this easily by moving the information into the narrative, like this:

“I was at Bleecker and Sullivan yesterday,” he said. Standing on the corner, he remembered that rainy night. They had fought over a cab and he relented. It was a good thing, too, otherwise they might not have married.

Or, you could imply more than you state directly, like this:

“I was at Bleecker and Sullivan yesterday,” he said.

She looked up from her book and smiled. “I should have known better than to marry you. What kind of guy tries to steal a girl’s cab?”

This doesn’t convey all the details of what happened, but the reader knows it’s a tender memory for them.

Always be careful of dialogue that sounds like narrative. When you’re writing, you have time to explain and work the prose so that it sounds good. But when people—and characters—speak aloud, they’re thinking on their feet, taking visual stimuli into account, and negotiating emotions. It’s particularly awkward to hear a polished line of dialogue when emotions are running high:

“I can’t believe you told my mother I lied to her. I feel so betrayed by you. And this, after all the times I bailed you out with your sister. I never want to have to tell her you’re not home again. Don’t ask me to do that.”

Rarely can we be so eloquent and complete. Factor in the shock, anger, and frustration and this is bound to sound more realistic:

“You told her? And I lied to Bea for you, twenty times maybe more.”

Keep your ears open to the conversations that surround you daily and you’ll see the way these issues manifest in people’s speech.