Humans often compare their happiness level to the happiness level of other people. Generally, humans believe that they know what happiness is better than what others believe happiness is. Dan Gilbert gives the example of Lori and George Schappell, two conjoined twins from birth. When asked whether they would prefer to remain conjoined for the rest of their lives or to be able to live separate lives, they answered that they would prefer to live conjoined, since it is what brings them the most joy. Many people responded to this in surprise, awed at how anyone would rather live conjoined with no privacy or individuality. In this case, we believe we know the true meaning of happiness more than Lori and George, since they "don't know what it is like to live an individual life." They don't know what true happiness is.
Scientific Explanations
In order to explain why we compare, or how we compare our happiness to others, Dan Gilbert uses the language squishing hypothesis. For instance, Gilbert uses the example of Lori and George to explain this. When asked how happy getting a birthday cake makes them feel, Lori and George answered 8/8. However, the average person would have answered a 4 or a 5 out of 8. According to the language squishing hypothesis, Lori and George feel exactly how the average person would when receiving the birthday cake, however, they are using the language wrong. On the other hand, this could be explained by the language stretching hypothesis. According to this hypothesis, Lori and George feel exactly the same when receiving a birthday cake than an average person feeling level 8 happiness, however, they rate that experience as being an 8 because they haven't experienced the same things as the average person. As they experience more things, their scale might change and adapt to their new experiences. Both hypotheses could be used to describe why everyone rates different experiences as being different levels of happiness.
How does this impact our happiness?
As you can see, happiness is purely subjective. This is why, studying emotions is incredibly complicated, and quite frankly, not very accurate. This is why there are three considerations to be considered when talking about happiness and comparing it to others'. First of all, all methods of measuring emotions is imperfect. This should be taken into consideration when comparing our happiness to others'. Second, when taking this into consideration and understanding that emotions are subjective, some emotions can be understood better. Either way, using comparison with happiness is not accurate. Comparing your happiness to others shouldn't be an indicator of how happy you are. Similarly, there is not one person who "knows" what happiness really is. It is in everyone's subjective experience to describe it as they wish.