Why do some people use others for their own benefit?

Is it possible to be happy in a marriage with a partner who has some of these 12 traits that psychologists say are linked to chronic unhappiness?
Why does my boyfriend want me when we’re on break but then when we are back together why does he not want me?
Why do some people believe that "the one" doesn't exist, and how can we find fulfillment without waiting for a perfect partner?