There are casual acquaintances, nodding acquaintances, and, of course, friends who are in our hearts.

Then the sentence "You don't have to say everything from your heart" means that before you directly determine that some friends are close friends,

It is necessary to guard against people.

Or you can try to protect yourself without knowing each other.

There is no contradiction with "friends should be sincere"

Just need to divide different occasions and times.