Because we falsely believe everyone is as contemptuous as we are?
I certainly don’t want to make generalizations or assumptions about what’s going on cognitively/emotionally in others, but here’s my take:
1) Narcissists are prone to see everyone or at least the majority of people as an enemy (threat) or someone to be dominated (“subject”). Life is a test that must be aced, a competition that must be won. Defeat is not acceptable. We would rather throw in the towel and surrender on our own terms than be defeated. This attitude limits the ability for any sense of equality or mutuality in relationships. The parties cannot be on equal footing, because humans aren't equal in the mind of a narcissist.
2) Narcissists, particularly of the hypersensitive/compensatory variety, have a constant barrage of critical internal talk, typically targeted at others during periods of grandiosity but also at the self during periods of vulnerability/depletion. For me at least, it’s easy to assume everyone else is continuously mentally evaluating and “striking me down" as I do this continuously to others. This applies for friends, family, acquaintances, lovers, everyone... It’s the whole idealization->devaluation idea, which brings us to...
3) The habit of throwing people away casually, even those who apparently felt strong connections to the narcissist likely leads to a conscious (or unconscious) belief that we could be abandoned or thrown away at anytime. On the other hand, we also live in a constant fear of growing bored and disappointed with those we are currently relatively close to. Taken together, intimacy is inhibited, because of simultaneous fears of disappointment (which was necessitate abandoning others) and abandonment.
4) Just a simple dearth of empathy makes it hard to feel genuinely sad or happy for others, the majority of the time. This again, limits intimacy, and relationships become about mental stimulation, emotional intensity, and seeking admiration/attention, rather than the connection for the sake of unconditional, mutual connection...
Hopefully that at least helps explain the idea...
(From one anxious/obsessive narc's academic and personal point of view.)