Why is it so hard to measure happiness?

I've seen a lot of my peers mention this topic about measuring happiness, since its the most recent documentary that we watched, I honestly had never thought about being able to measure something as "happiness." It's true that we have a whole branch in psychology dedicated to studying depression and all kinds of negative mental states of the mind, but it would also make sense that if we have such diverse definitions and study fields for people with depression, technically we should also have another branch that classifies all sorts of positive emotions right? Well we don't. But I think that it's not because we don't consider happiness as something important, but rather it is much harder to measure the happiness that a person is feeling, since depending on the person his levels of happiness might be extremely different than mine. For example, it's true that whenever we see a friend that we haven't seen in a while the first thing we ask him might be "Hey! how you've been?" and the obvious response from them will most likely be "Good man! How about you?" and of course you may also respond with another lie, "Good, Good." Lying about our emotional state is one of the most common things we as people tend to do in order to prevent involving other people into our personal life. But is this really the way that things are supposed to turn out to? For example, I feel the most happiest whenever I'm in Mexico, since I've always considered Mexico as my one true home. But whenever I'm here in the US I feel impatient to be on my way to Mexico again! Measuring the levels of happiness that a person can have is definitely more challenging than measuring their levels of depression, since happiness can easily be imitated.. whereas depression cannot.