Term for "constantly unsatisfied soul"

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.

Solution 1:

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:

blue, complaining, crabby, disgruntled, displeased, disquieted, exasperated, fed up, fretful, griping, perturbed, picky, restless, ungratified, upset, vexed

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.

Solution 2:

If we're looking for a noun, I'd go for malcontent, defined as a person who is dissatisfied and rebellious. I think this epithet particularly identifies a person who will always find something to complain about, no matter how much effort others put into improving things for him.

For adjectives, @drɱ65 provides a goodly slew of alternatives. Interestingly, I don't think "malcontented" would sit well with those. I think it's not often used to describe the type of person I would quite naturally call a "malcontent", which for me only seems to work as a noun.

Solution 3:

insatiable |in-sey-shuh-buhl|
adjective
Not satiable; incapable of being satisfied or appeased: insatiable hunger for knowledge.

http://www.google.ca/search?sourceid=chrome&ie=UTF-8&q=define%3A+insatiable

Solution 4:

I would supply the context for a better answer. 'Curmudgeon' might work depending on what you really mean, as might 'fussbudget'. The former is someone whose expectations the world never meets, and who as a result has adopted a demeanor of grumpy forbearance, while the second is someone who takes joy in finding fault with everything, no matter how slight. A 'malcontent' can be someone who is very angry (at, say, a government), whereas a fussbudget is more often someone ceaselessly harping on trifles, and curmudgeons are often portrayed as secretly optimistic, despite their endless disappointments.