Well, I know I may be new here, but I hope you can forgive an old busybody for butting into your conversation.
Now, I've spent a lot of time reading out of a lot of different philosophers, but I never was the academic sort and I can't manage to present their views in any real way, only my own view as affected by having digested them.
It is the Cartesian assumption as well as the Socratic one that you can't really know anything for certain, outside that you exist, from Descartes's famous "I think, therefore I am" and Socrates's "The only thing I know is that I know nothing," and I apologize if I mukked up those quotations, but bear with me. The point I make is that the world and reality is ultimately what you make of it, and that your willpower is the one thing you have full control over, and with it you affect your experience.
An event or ideal is distressing only if you let it be so, and with enough willpower you can overturn that distress, and in a similar manner you may garner joy from other things in life. Of course, as often as I make this point to people, I still can't fully invest in it.
The more time I or anyone else spends dwelling upon existence the less sense it starts to make, and the more horrid the great looming unknown becomes. There is an old saying: "ignorance is bliss," and I'm afraid I can't credit that properly, but it rings true. I may take joy in hearing a song, but if I learn that, say, that song had been stolen I may be instead angry. If I hear the energy I used to have my device play that song could have been better used in life support systems, I may feel guilty. If I learn that the device I hear the song through is in mild danger of detonating from internal malfunctions, I may feel frightened.
I know it sounds damning to put happiness in this context, of something only to be had by those who will not think past the moment, but I do not mean to make that sound like a bad thing. In a way, it's preferable. There is so much we cannot change with our insignificant existences, and to worry ourselves with things beyond our power is a good way to severely cripple your quality of life--take it from me, if you care to lend me any credit.
I suppose that given all that, happiness, to me, is the acceptance of one's place in reality, and the realization that one should revel in the power they are allotted, not curse their luck for not having enough to fix the world's problems. It's finding one's place, and understanding that they needn't be a hero or villain or legend or monster, they need only be themselves.
Sappy, perhaps, but a good deal more than a lot of people have ever achieved.
