Dale Carnegie was a teacher in public speaking and Warren Buffett once took his class that to this day Buffett still admits it was one of the best classes he ever took in his entire life. A wonderful and concise quote from Mr. Carnegie:
Success is getting what you want. Happiness is wanting what you get.
The aim to succeed in life may not result in happiness. That is true when you think deeply about it. For instance some of the richest people on the planet are on the surface very successful, but that does not mean they are happy. Those who choose to be happy will always appreciate what they have instead of what they want to get.
The original 2010 study, conducted by Princeton University researchers, found that people's day-to-day happiness increases with income up to about $75,000 at which point it tops out. So according to this study, someone who makes $100,000 a year is not happier on a day-to-day level than someone making $75,000. The 2010 study said this is largely because people at this income point don't have financial stresses such as whether they'll have enough to pay rent each month. However, while the 2010 study found that day-to-day happiness eventually tops out, it also found that general life satisfaction does not and, indeed, continues growing alongside income.
There are studies done through the years that proves people in general after earning a certain amount of income with any additional increase will have incremental change in their happiness. Not to say this is with all people, but as an average.
Another way of looking at it is many people are in the rat race to make the most money during their entire life. Meanwhile they miss out on spending enough time to enjoy their lives. Example is spending time working instead of spending time with family and friends. The end result is they may succeed in achieving their goals but it may not make them happy.