Happiness is a state of mind. What's actually necessary is a sense of purpose.

That's why people in vastly differing situations can both be happy, because it's a subjective function of how much meaning you have in your life. Having a purpose lets you battle through depressing lows and accomplish great deeds and still remain fulfilled. Otherwise it's just a chase for the next big thing, and while that can be fun for a bit, it also leaves you empty and lost.