When it comes to happiness, I do believe it can last forever, as long as fate allows it. If you are with the right person, and naturally you should know if they are "the one", your happiness together should very well last for as long as you live. This is a great question!
From Buddha to John Locke, people have always realized that struggle is the fundamental attribute common in life. While the relative peace and prosperity of the past 60 years has made people re evaluate their views on the world, the ultimate core of the issue is the same - happiness is temporary, but that doesn't mean its constantly out of reach.
I do not believe that emotions, such as happiness, are permanent. I believe emotions are something that is fleeting and can change frequently for any given person. I do not believe it is possible for someone to be happy all of the time or feel happiness all of the time.
That is the bad news. Everything changes and happiness turns into sadness. The good news is that the opposite is also true. And more good news is that there is a state we can call joy that is different from happiness and that can always be accessed inside of us. It doesn't depend upon externals.
Happiness is a lie that just exist inside your head to fool your mind in to believing you are ok buy we never will be truly ok; at least i wont. Its not about hating the idea of being happy or others capable of being happpy, but you are never always happy.
While a lot of people argue that "happiness is a choice," more recent studies tend to show that people are happy or sad just in general. We all tend to have a different spot on the scale where we sit; i.e., some people are overall more happy or sad than others. If something good happens or something bad happens, we will temporarily either go up or down on that scale, respectively. However, over time, we'll eventually level back to around where we were before. So people who are happy tend to level back to their normal state of happiness, while those who are more sad level back to that place.