Some narcissists will use their romantic partners to meet unreasonable self-serving needs, fulfill unrealized ambitions, or cover up self-perceived inadequacies and flaws:

Some narcissists, being highly self-centered and self-absorbed, expect instant gratification to fulfill their needs. 

Many narcissists can come across as alluring and attractive, especially during the initial stages of a relationship.

A narcissist will frequently react negatively when you don’t give them what they want, in the way they want it. Some of the common responses include:

Some narcissists will constantly put other people down, to boost their own desirability and acceptability. 

Many narcissists lack reliability and follow through. This can range from regularly breaking appointments, to habitually falling through on promises and agreements.

A pathological narcissist loves to talk about himself, often in exaggerated and grandiose terms. 

Narcissists often expect people to cater to their needs, without being considerate in return. In their mindset, the world revolves around them. 

What distinguishes a narcissist’s lack of commitment is his or her desire to keep the status quo with you, reap the benefits of intimacy, while keeping an eye out for other, perhaps more eligible (in the narcissist’s view) prospects.