Is there a single word or phrase which can describe a person who is always slightly unsatisfied?
Update: To provide more context, This person is slightly unsatisfied with all things of life. Does too much work and still thinks he is doing less. Takes up a hobby and quickly lose interest wanting more.
Best Answer
Discontent or discontented could easily fit.
Here is a list of several words that are somewhat synonyms to this concept, and depending on context might prove useful:
It depends partly on how active the discontentment is. Exasperated, griping, crabby, etc, have connotations of outward display of discontentment, while blue, discontent, disgruntled, etc can be relatively contained.
Restless presents an important nuance: it does not have to be negative. This is a valuable word to use when the discontentment is not a bad thing, especially when there is hope of resolving the cause of restlessness.
Perfectionist might be a helpful word (also maybe stickler or idealist); this describes someone who can't relax until the thing he/she is working on is perfect.