The answer. Because it serves you.
It is inevitable that the narcissist in your life will irritate you. A narcissist won't listen, will talk too much, will never ask what will work for you. You might even be feel crazy because the narcissist in your life will quite often neglect to pass on information that you might need to know, change plans without clueing you in, promise to handle things and then only do it half way or not at all. A narcissist will do stuff for herself/himself but manage to convince you that it's being done for you. This little list just scratches the surface of ways in which you can get triggered emotionally when interacting with a narcissist.
Some of us are infinitely easy going, and it can take a LOT of those sorts of interactions before we start to get peeved. Whatever your tolerance level, once a self-absorbed person starts to bug you, it's kind of like fingernails on the blackboard -- you notice. You may not know that the things that bother you are narcissistic behavior, you probably just register that this particular person does stuff that annoys you. It takes a fair amount of frustration before you try to sleuth out what's behind your annoyance. By the time you begin to figure it out, there's a strong likelihood you've had enough accumulated experiences that you are easily triggered.
When you get triggered, it's just like that -- a trigger is pulled, and you find yourself catapulting from a decent mood to pissed off. It's possible you've learned to keep quiet and process this upset before responding, but not all situations lend themselves to this sort of mature response. If you're at work, the demands of the situation may require something more immediate. You may not bring all your emotional intelligence skills with you all the time. It'd be nice if you did, but stress and a fast pace can short circuit your best intentions. Your mouth pops open, and what you REALLY think comes tripping out.
Certainly, you've learned the techniques for handling anger. Things like counting to 10 before you speak, taking deep breaths, processing your anger off line and speaking only after you've calmed down and sorted things out. You probably know that this is considered more professional, or more wise. But, when it comes to interactions with a narcissist, there are some extra, compelling reasons that might help you remember to use all those skills.
A narcissist needs attention, and negative attention will provide for her needs just as well as positive attention. A provoked narcissist is much less predictable than a happy, validated narcissist. Once you let loose, you have no idea what will come next. A provoked narcissist is not a nice person, to say the least.
Another thing to know about a narcissist is that she takes on the emotions of people around her. She doesn't know she does this, but she does. It's due to being enmeshed with others rather than having a separate and distinct sense of self. So if you are angry, you are likely to provoke the narcissist into being angry just because you are (although the narcissist may come up with other stories about why she's angry; often they are non-sequiturs that end up distracting you from the issues at hand).
So keeping your cool despite your frustration, annoyance, anger, or rage serves you in a big way. It keeps you from being a source of sustenance for the narcissist. It keeps the narcissist calm and more predictable. It allows you to keep the situation from escalating. Although you may be upset, refraining from voicing it allows you to maintain at least some level of calm so you end up with fewer unintended, unanticipated results.
This also gives you time to shift your attention back to your own reality. The inner framework of a narcissist is one in which you exist to do for the narcissist. It is confusing and debilitating for an emotionally healthy person to exist in that reality for long. Engaging in your anger with the narcissist keeps you in their reality, and ironically keeps you supporting the narcissist. If done long term this can be isolating and undermine your well-being. So all those ideas for managing your anger are especially useful when you're confronted with an annoying narcissist. Take a deep breath and get some space as quickly as you can.