For many people, happiness has become their goal/purpose in life.
Finding happiness has been everyone's struggle.
To be honest, I don't think anyone has actually found it, which is why we associate happiness with things like success, partner/spouse, children, wealth, health, beauty, etc. Because we will be happier when we are successful, we can be with our loved ones, we can buy whatever we want, we are strong and healthy, we are good looking.. (than when we fail, alone, poor, weak, ugly..).
But how many people in this world are successful? How long can our loved ones be with us? Even the richest people on the planet couldn't buy anything they wanted. How many people have never been weak and sick? Can we always be handsome/beautiful?
When someone associate happiness with temporary things, his/her happiness is also temporary. This means that in the end, his/her goal/purpose in life will never be achieved. Because our life is also temporary, so it can never be the true goal/purpose, we are not the center. If someone associate his/her goal/purpose with him/her self, there will be no goal/purpose after he/she dies. One's happiness is not the true goal/purpose.
Summary:
If we are unhappy, it doesn't mean our life has failed.
If we are happy because of temporary things, remember they are only temporary.