Well for me, sometimes my mouth kicks off before my brain has had time to kick in. lol
They want to manipulate your feelings in some way. Sometimes they can't accomplish that with the truth.
It's not always such a bad thing. Sometimes it can mean that someone wants to make you feel good, so they overinflate a compliment. Or they exaggerate a story to make it funnier, etc.
save face, positively or negatively, same reason people lie
To make you happy ?
They are idle promisers and self-centered
Sometimes they're just trying not to be rude. Other times they're just ********.
I believe some people do that because they may be unhappy in their lives and venting is more important at that moment. Other times it can be due to envy or jealousy. Allowing emotions to get out of control in general tends to put temporary blinders as to the consequences. Kinda like what Icarus2006 said.
Because they let their emotions rule them at the time. Because they have no filter. Because they want to hurt.