Happiness is a (temporary feeling) of joy within.
Essentially everything is temporary. Tomorrow you could get news that a loved one has died and the happiness you feel (right now) will have disappeared. That's life!
It's important to keep things in perspective. Enjoy the "good times" when they come but be aware that no one gets a life filled with only sunshine and rainbows. Life is a rollercoaster ride.
The great thing about accepting moments as being temporary is you realize "bad times" are temporary as well! Being too "unrealistic" or too "cynical" can lead to heartache & depression.