"Giver" Vs. "Doormat"

A "good" relationship, whether it's romantic, friendship, or family, is one where there is MUTUAL support. If one person is being supportive of the other, and of the relationship itself, and the other is taking that support but not actively giving any back, then it's not a mutually supportive relationship, and the first person is likely to burn out or leave.

In relationships with Narcissistic people, this one-way dynamic is compounded by the N's habit of isolating the other person so they don't get support from anyone else, either. Also, N's want extra and more support than other people do, they want all compassion and allowance for literally anything and everything they do and say, they want to own the other person and use their energy, care, and strength, (and other things as well depending on the individual N.) and then want to "keep it", not giving any back, or giving it in drips and drops to keep the other person believing that they really do care, but then taking it away on a regular basis because they have to have "control".
When the "giver's" energy is depleted, there is no one to help them fill back up.

In other words, the "giver" is the one who is there when the other is upset, or is having a hard time, is sick, is dealing with hardship, tragedy, or loss. They are there being supportive and encouraging about the other's work, goals, and ambitions, and supportive of them personally and publicly. They stand up for them against detractors and rally support from others.
The "giver" is tolerant and even understanding of the other's emotional expressions even when they're very angry, and tries to understand and figure out the reasons behind the other person's actions that are not kind, respectful or considerate toward them. The "giver" tries to find ways to improve the relationship, heal themselves and help the other person. The "giver" does not desire to cause the other pain or hardship with abandonment, rejection, or abuse, and is quick to apologize and make amends when they've misstepped, or caused pain or sadness. Since the "giver" is not arrogant, they don't assume they have all the answers; they look for support, advice, and information from outside sources.

If both parties were doing these things and had a similar outlook, then the relationship would likely become healthy and solid very quickly. These things that "givers" do are not unhealthy, they are peace and solution-oriented. They become unhealthy when they are doing these things in a relationship with a person who is not interested in doing any of these things, in "giving back", in mutual support and compassion, in mutual respect, or in the other person's well-being, needs, and goals. They may want the "giver's" compassion, tolerance, care, respect, attention, love, and understanding very much, but they really are not interested in giving it back, or learning what it means to give it back.

Often, the "giver" is seeking a genuine connection with the other person, but the other person doesn't really want that, even if they say they do; they see "connection" as a tether where they become tied down with "obligation" and have to monitor and restrain their behavior, especially when it's with someone who is the opposite sex and/or a different age group. (Many narcissistic people only want "connection" with people who they identify with very, very closely; others they just want to receive things from, from care and attention to service to material goods.)

There are levels of Narcissistic relationships, and they take different forms. Some N.'s want the other person's emotional support, respect, attention, help, or praise, but have no intention of giving it back.

As the "level" increases, more and more things can replace "emotional support and respect", or can be added on top of that. For example it's typical for a sociopath Narcissist to want a person's money, but have no intention of paying it back or supporting them in any way.