You don’t withhold your feelings for the other person out of kindness or care.

You do it for yourself: to void future abandonment and to cover your ass. That way, no one can ever blame you.

Withholding is a form of control.
You present a “fake you,” which is manipulative, and when people discover the deception they leave, robbing you of your stability. Or they stay, and you resent them because you have to keep pretending.

(From Nick Pollard, mostly)