There is a concept in the field of psychology used to explain the effect you are referring to.
It is called the hedonic treadmill, or hedonic adaptation. It basically means what @GoodPersonEffed wrote, namely that feelings, both positive and negative, tend to change and eventually fade over time.
As far as negative feelings are concerned, the hedonic treadmill is absolutely crucial to our survival - without it, we would not be able to deal with circumstances which elicit strong negative emotional responses. Simply put, we would not be able to get over break ups, disappointments, death etc.
When it comes to positive emotions, happiness being of course the main one, the hedonic treadmill could be either a result of the evolution of our brains, or genetically wired into our cells.
Humans are unble to adapt to a fixed level of happiness. We are always chasing the next high.
This could account for why we are constantly improving ourselves. Imagine how little progress there would be in the world, if humans wrre not driven by the impetus to refocus their personal goals and desires, and to always strive for more.