Truly Superior

Perhaps the number one thing to keep in mind about Narcissists is that they honestly, literally, fully believe that they are BETTER THAN YOU. As in categorically. They believe they are a superior KIND of human being, and you're NOT. You are not one of them, you're just not, and you never will be (in their minds).
Why do they talk to you, then?

Well you might be a relative, so they're "stuck" with you, but you can see their disdain by the way they don't invite you to things, or constantly criticize and correct you, or leave you out of conversations and decisions, or treat you like you're incompetent, unstable, or just not good enough.
They might have mistaken you for one of THEM when they first met you, so they wanted to hook up with you and rule the world (according to THEIR plans, of course).
They might see you as a way to improve their IMAGE by association with you.
They might want to own you, like a pet, or a slave.
They might be fixated on having sex with you.
They might only talk to you when there's no one else around.
They might want something you have, anything at all, from money, to your car, your house, your family, your career, your reputation, your "power" (if they think you have some). It could even be your dog, or your friends; who knows.
They might want to get into places and associate with people that you have access to.
They might want to be YOUR "sidekick", and for you to be their "protector", parent, caretaker, bodyguard, provider.

~(If a Narcissist is reading this post right now, their reaction will most likely be eye-rolling or some kind of invalidation like "Oh brother, who cares?!", or defensive justification of the behavior they recognize like "Everyone does that!" or "Some people really ARE 'losers'!", or they'll adamantly deny they do any of these things, or ever have even come close to doing any of them, and could never, ever, ever, and anyone who does do them is EVIL and should be burned at the stake, or completely insane and should be locked away forever (deflection/projection/extreme reaction/lack of objectivity or empathy).)

Lack Of Self Control and Narcissism

If you can picture something you have a "weakness" for~ such as chocolate, or other sweets~ when you pass by the cookie tray, you know you "shouldn't", but you take one anyway. Same with other things, like breaking certain rules even though we know better, like turning right on red at a light that says "No Right On Red", or speeding, or bringing candy to the movie theater.

Now apply this "weakness" and "rule breaking" habits to relationships with PEOPLE. That's how Narcissists think.

"Well it's just one cookie".... Well, it's just one dance, just one sexually charged embrace, just one kiss, just one french kiss, just one bodily fluid exchange... no big DEAL... (as long as they get away with it... no one's going to find out, right? No harm no foul? What they don't know won't hurt them?...)

"The cashier didn't charge me for those socks, well it's just a couple of dollars".... Well, I didn't pay her back the money she loaned me, but I will eventually, when I get around to it, when I get some extra money, when I have nothing I want to buy for myself, it's just a few thousand dollars...

"It's normal not to tell your partner every little detail of your life when you're apart, you don't have to tell them every single thing"... Umm... I didn't tell you about buying the boat because I didn't think you'd mind... Umm... I didn't tell you about the party because you don't like that kind of thing.... Umm... I didn't tell you about the vacation with my "buddies" because, umm, you probably wouldn't want to go anyway... Uhh.. I didn't tell you about going to the bar with her/him because you don't really like that kind of atmosphere... Oh, I didn't tell you about the school play because I figured you'd be busy...

Because of their poor self-control and weak boundaries, many Narcissists become habitual liars in continuous covering up of their behaviors. They can become so habitual that they lose their awareness of the act of lying and fabrication, and end up living in a world of delusion that they fully expect others to SWALLOW. Amazingly, if someone does NOT swallow their Tall Tales, and even if they have irrefutable proof and several witnesses, the Narcissist will become ANGRY AT THEM. It's as if the non-believer is doing a HOSTILE DEED to them by "bursting their bubble", destroying their illusion. There is NO remorse for the trouble, disaster, and often devastation they have caused, their only emotion is about getting caught, and getting called out.