I hope that most readers here are like you and can distinguish between the extreme sadistic fantasies enjoyed on CF of unremitting pain and torture for a slave, who has no worth and is treated worse than a beast of burden, and the reality of consensual slavery.
In the latter, the slave has consciously or unconsciously chosen to transfer power and choice to their master. Yes, they do this in part from a masochistic desire for abuse. But also there, whether explicit or not are two other motivations.
First, a desire to not have to make choices and decisions. We do not all crave above all our own way. Some desire to just go with the flow of another, wiser person's orders.
Secondly, and part of the first, it is the desire to trust, to recognize another who has your best interests at heart. Not just your sexual desire for punishment, but all your needs as a person. It is closely akin to the child's willing and eager submission to a loving but firm parent - or the faithfuls' prostrate obedience to their God.
This kind of slave/master relationship can be fulfilling and very healthy for both partners. If, on the other hand, the relationship leads to ill-health (mentally or physically) on the part of one or both, the relationship needs to change or end.
{- Master}