For the same reason, they could fall in love with anyone else.
People can fall in love with narcissists just the same way they fall in love with anyone else. They meet, they are attracted to them, the person has qualities that they like, and they feel connected to the person. Of course, this does not mean that the person with NPD will handle the relationship’s ups and downs like anyone else, but usually people do not realize this at the beginning. Even when people are exposed to the red flags that they may be involved with someone with NPD, most people ignore these signs, and focus on the aspects of the relationship that they like, feeling hopeful that it will still be good, it will get better, or they can somehow love the person enough/ change them/ or “be worth” the person changing. While these are common desires, many people come to realize that they can not change the person, manage their mood swings, and/or find it incredibly hurtful when the person with NPD no longer treats them like the shiny object that they were at the beginning of the relationship.
The point is…
People fall in love with narcissists for about a million reasons. Many narcissists have other appealing characteristics like intelligence, good looks, humor, power, career success, and outward confidence. While others may not have any of these positive traits. Some people may “fall in love” with someone with NPD due to unconsciously repeating maladaptive patterns from an earlier stage in their life, while others may simply fall in love and weren’t paying attention to the red flags at the beginning of the relationship due to all of the intoxicating feelings of falling in love and wanting this person’s affection.
Originally published as an answer on Quora.com.