I think people get pulled in, like Jefftele says, because of one's own needs and also because other people are just as "complicated" as we are, they're not all one thing to the same degree all the time. Narcissists can be quite charming or they wouldn't be able to exploit others and one could feel bad if one knows someone with weak self-esteem, it is nice to think we might be able to help another; there's a little bit of narcissism in everyone (why do you think therapists are therapists?), it's a continuum.

I think the trick is kind of scary; to think of one's self first (but the narcissist doesn't need to "think" of themselves, it comes too automatically) and look at the whole relationship, etc. It's easy to see a narcissist if you look at the whole relationship and realize that you've never gotten anything from them, only given. Taking a hard look at one's self, one has to decide if the feel-good of giving is worth the pain of never getting but being used instead. If there is no sincere "Thank you!" after giving, no attempts at repayment, it's hard to feel good giving unless there's something not quite right with one's own self/world. If simple requests are ignored or excuses are flimsy at best, that's a loud-and-clear warning sign.