And that's the thing about people who mean everything they say. They think everyone else does too.

This quote means that people who are always honest and sincere believe that everyone else is the same way. They think that everyone else also speaks from the heart and is always honest and genuine. However, sometimes this is not the case and people can be deceptive or insincere. Therefore, it is important for these sincere and honest people to be aware that not everyone is like them.