Narcissistic people are very needy for reward and often don't think about the consequences. They use you when they need the admiration, and put you back on the shelf when they're full.
The reason they can be warm and sensitive one minute and turn into ignorers the next, is because it's not a matter of intimacy, but of control. Their extreme changes have a very bad effect on the hormonal balance of the brain because when you are in their world, it is flooded with dopamine and oxytocin.