Our "flesh" takes over and we often say things that we "don't really mean" ...
Or even things that we can't believe we said ourselves.
This is because the "tongue" or our mouths are like a portal or window to our sinful flesh, allowing it to manifest itself.
But through Jesus Christ, and His Spirit which dwells in us, we do have a way to overcome this sinful nature in us all.
"And the tongue is a fire, a world of iniquity: so is the tongue among our members, that it defiles the whole body, and sets on fire the course of nature; and it is set on fire of hell."
"But the wisdom that is from above is first pure, then peaceable, gentle, and easy to be entreated, full of mercy and good fruits, without partiality, and without hypocrisy."
But the tongue can no man tame; it is an unruly evil, full of deadly poison.