I've been diagnosed as having narcissistic personality disorder (NPD) by several doctors so maybe I can shed some light on this.
NPD is the inability to generate and/or experience internal validation. It is the lack of internal voice saying "good job". NPD is not self-aggrandizement although that is a common coping mechanism used to deal with the underlying disorder. I don't think I'm some sort of minor deity, but I'm also told by my doctors that I'm considerably more introspective than the typical narcissist (the irony of my docs being a source of praise is not lost on me).
Because I do not have an internal source of validation I seek out validation and praise from others to fill the void. Psychiatrists call these sources of validation "narcissistic supply". That supply is a finite resource, eventually I either exhaust or build up a tolerance to the individual / group supplying the validation which causes me to seek out new sources. Some people have compared it to an addict trying to find their fix, a metaphor I find distasteful since it does people suffering from addiction a disservice.
In this way I don't think I can be happy because I cannot remain satisfied. Sometimes I feel good, but I understand that no matter what I do, success I achieve, or people I help it will never be enough. I have a hard time staying still because I know that whatever source of supply I'm currently tapping will eventually run dry. It's a rather restless experience.