If you are nice to a narcissist and he likes you why at times should he ignore you?

Narcissists don't see other people as humans with feelings, but as objects whose purpose it is to gratify their intense need for praise and recognition. Narcissists tend to treat others nicely only when it suits their purpose -- for example, if you're giving needed ego-strokes, they're performing for an audience, or they perceive in you something special that enhances their status. If someone -- or something -- else is filling their narcissistic supply, they may suddenly see you as irrelevant and ignore you.

Sad to say, narcissists lack genuine, warm, reciprocal feelings for other people and usually make lousy friends and partners. Their primary interest is in themselves and always will be.