If consumed long enough with any task or activity a thoughtful person will eventually start to question meaning and purpose, even if consumed by free choice, that is to say, even if the activity is engaged in by a person in times of leisure or 'free time'. While under investigation, there can be a reaffirmation of the intrinsic value of the activity and meaning is successfully perceived or the opposite can be revealed - an emptiness will be sensed, and a confused unrest will start to grow within. This may not even be a conscious process, I have oft times experienced it vaguely and subconsciously, which furthermore leads to a general discontent and (again) vague unrest in my being. Further investigation of these 'vaguenesses' will lead to the realization of the emptiness. It is one thing to say "I am happy" and "I am fulfilled" but it is another altogether to actually be happy and fulfilled. Pressure from exterior forces (jobs, people, perceived expectations, a want or desire to be something other than we are, etc.), our own personal motives, and dishonesty within ourselves to ourselves can lead us to feel confused as to how we actually feel.
But, the thoughtful and intelligent human (perhaps even spiritual?) will always, eventually, be brought to critical thought even if never conscious of any unrest or even if in a state of consistent 'vagueness'. It can happen when 'zoning out' - washing dishes, standing in line somewhere, walking, running, or any activity in which the active mind is subdued and the always constant background thought takes over, as if in daydream. This is, seemingly, how insight occurs. Thought seems to come to us from 'beyond', but in reality we have been thinking, unaware, the entire time. Lonergan, in his tome "Insight" would have much more to say on that and the human condition of thought in general, though I am afraid my feeble memory does not recall his thoughts offhand (it has been 4 years).
Now, a human can either embrace this emptiness and continue on, now aware of the vague unhappiness, which in it's realization can become solid, or, perhaps even realize the emptiness and continue in a masturbatory manner, with pleasurable manic highs followed by depressive lows. Still, disillusionment pervades.
However, if the activity truly brings no intrinsic joy there is eventually a recoil from the emptiness and a shift towards meaning. Whether a person is capable of making this shift (ie if the source of unhappiness/that which is inherently empty/that which is devoid of meaning is a task or activity which one is not forced to complete) is an issue, and if they are forced to continue (eg: at a job) then we have the makings of intense despair.
Though, it seems as if the truly 'freed' person (a phrase of which deserves it's own book to explain) would say that no man is "forced" to do any-thing. Which is true (idealistically, in the modern sense). However, when one falls into certain conditions and finds himself in a specific situation, it becomes very hard to break out of that conditioning. Jobs, for example. It is hard to leave any position that upon hard work was earned and simply seek out a new one, especially in a new field. There are, as mentioned before, these pressures. Parents pressure us to get an education, spouses to make money, society to succeed. But what seems to truly matter is personal happiness, which is not as self-serving and selfish as it sounds (radically different from Randian objectivism).
Personal happiness then, briefly (because I have to get ready for this day) seems to involve feeling fulfilled. (This word is not thrown around with levity, and if given the infinite time and peace of a life unconcerned with money and computers and girls and work I would certainly read and write at great lengths about it and many other things as the philosophers of old). Personal happiness is not just about being selfish; doing just what pleases yourself. Quite the contrary, and if one's life is modeled around selfishness then the very things I have just written would plague you. Emptiness indeed! Rather, personal happiness seems to involve feeling as if you've benefitted someone or something, made a connection to something, perhaps even aided the common good, or improved the human condition. These seem to be things that encourage feelings of fulfillment.
And I don't believe this is a utilitarian version of doing things to feel good. I believe in the truest good that actually comes out of them, and as a result makes one feel fulfilled, which is to say, fills one with a sense of meaning.
I'd like to work out the thoughts in the final paragraph more, regarding personal happiness, because it isn't just about fulfillment and the common good, though that is certainly an aspect. However, I need to go (time, cursed time, always time).