Why Is Happiness So Hard to Define?

What exactly is happiness, and why does it seem different for every person? Is happiness simply a temporary emotion, or is it more of a long-term state of being? Why do some people feel happy with little things, while others keep chasing more but never feel satisfied? Could cultural values, personal goals, or even brain chemistry influence how we experience happiness? Is it possible to measure happiness scientifically, or will it always remain subjective? And if happiness is the ultimate goal of life, why does it often feel so difficult to achieve and maintain?