you and Sam merlotte have a emotional connection between eachother as Whenever you cry he cries to and vice-versa. Also sam is a shifter which means he can shift into any animal. While your a full-blooded vampire. Also since your the only full-blooded vampire around no one has seen what your tears look like when you cry. Well your tears are blood since only full-blooded vampires cry tears of blood which sam only knows. Also since sam is a shifter his tears are normal whenever he cries