Because as the saying goes, the grass is always greener on the other side,
No matter what you have in life, it is never enough. That is the curse of humanity. We always want more, what we perceive to be better. We justify it by saying that we are improving ourselves, improving our lot in life, etc but we are forever chasing that pot of gold at the end of the rainbow.
A vast majority of people in the world have difficulty accepting what they have and not wanting more. There have been many psychological studies done on this and in the end most determine that mankind is simply not able to be content or happy for long periods of time. We are constantly driven to want/need more, to never settle.
In most areas that is considered a good thing. Wanting to improve your earnings, your education, your skills, your understanding of things, gaining a new skill etc all of these are considered positive attributes and usually fall under the heading AMBITION/AMBITIOUS
However when it comes to love or relationships these same actions are suddenly negative. And why is that? Simple, it involves other individuals, it no longer just effects you, but can turn someone else's life upside down, cause emotional distress, etc.
As a couple grows and matures and evolves through their relationship it is not done at the same pace. One individual always falls in love faster than the other, stays in love longer than the other, is willing to put up with the others foibles and annoying habits longer than the other,etc.
It is this difference in speed over the evolution of the relationship that causes one person to lose interest faster than the other. Once interest in lost in the current partner, it is usually switched to another prospective candidate. It really doesn't matter if that individual shows interest as most people believe themselves to be able to "make" someone interested in them. So while they might be interested today, surely they will be soon, if it is "worked" upon.
It is a tough life lesson to learn and experience and if you are asking this question I am sorry you are going through it, but be assured, it will get better and you will meet someone at some point whose interest in you will not wane through the years. Hopefully, at that point, your interest in them will not diminish either.
The age wanting what you cannot have. Human nature. Treat them mean keep them keen is actually very relevant to this