Because evil pixies come and steal the good feelings, and then eat them. They do this in the hope of turning the entire population into emos. (They think the colour black is cool, and so want more people to only wear that one colour.)
However, what the pixies don't know is that their droppings - which contain all that eaten happiness - floats through the air, and is breathed in by people, making them happy again. (Pixie droppings are a very potent happiness drug. Rehab is filled with people who have accidently inhaled too much, and are now addicted to it.)
In essence, the evil pixies are unknowingly eating their own poop, over and over again, in their vain hope of stamping out happiness forever.
And that, children, is why positive feelings are only felt fleetingly
Sureal wins. I was going to say because in the moment you're probably a lot more excited than you are truly content, and then that feeling goes away, and... you're back to wherever you started.
"You laugh at me because I'm different. I laugh at you because you're all the same."
Oh, I know this all too well. Because the world is a dark place. So many people have shed their imagination and are motivated only by their selfish desires. Money and many other worldly things add fuel to this. Happiness is what shines a light in the darkness, but the depression induced by the corruption around us has a vendetta against what puts a smile on our faces.
At least, that's how it is were I live. Very rarely do I smile, and when I do it's never for very long.
"Jesus said to her, “I am the resurrection and the life. The one who believes in me will live, even though they die; and whoever lives by believing in me will never die. Do you believe this?” -John 11:25-26
Because people think the wrong things will make them happy. It's like with drugs: you take crack to get high, but you go into a slump afterward. So you take more crack. Unfortunately your body starts to get used to it after a while, and you have to take more and more with less and less results. When you try to achieve long-lasting happiness through something like personal possessions, or a job, or a hobby, even to some extent through your expectations of other people or 'finding your niche', you're only going to make it harder for yourself to be happy.
"Indeed the safest road to Hell is the gradual one-- the gentle slope, soft underfoot, without sudden turnings, without milestones, without signposts." --The Screwtape Letters by C. S. Lewis
Because everything is always changing - people, places, situations, relationships, etc. Even our perception of happiness changes. Let's say Suzie buys a puppy. She's so happy that she finally has a puppy. She feeds it and walks it and plays with it. But over time, the joy that she felt when she bought the puppy begins to fade. Perhaps because she's gotten used to it (so her mind has adjusted - her own mind has changed), perhaps because she doesn't have as much time to look after it as she did at first (her circumstances have changed), maybe because the puppy is getting bigger and isn't quite as cute as it used to be (the puppy has changed). Things just change. And if everything is always changing, including ourselves, how can we possibly expect the same thing to always make us happy?
"Jesus said to her, “I am the resurrection and the life. The one who believes in me will live, even though they die; and whoever lives by believing in me will never die. Do you believe this?” -John 11:25-26