Having grown up in a difficult environment, a negative emotional climate is experienced as normal and familiar. This is why there is often little recognition of disrespect. If their partner is angry or upset, the codependent individual will implicitly assume that they did something to cause the anger. It does not occur to them that it is their partner's responsibility to deal with their problem and to treat others respectfully. It does not occur to them that it is their responsibility to themselves to stop another person's demeaning behavior toward them.
Disrespect is normal.
An unfortunate side effect of the codependent person's willingness to ignore, excuse, or otherwise allow the partner's abuse or disrespect, enables the misbehavior directed at them to continue and intensify. Because codependent individuals are approval-driven, they cannot stand it when others are angry at or disappointed with them. As such, they unwittingly place themselves in a position to be taken advantage of. The more approval is needed, the less likely is the individual to realize the extent of their self-sacrifice in favor of tending to the needs of the other. Targeting the appropriate person may jeopardize a source of approval and self-esteem.