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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

 The interest is a motivation that is transmitted when wanting to obtain something not reached or that requires an effort to achieve it.

 That is transmitted in the human being mainly in the curiosity of knowing everything and possessing it since it represents something unattainable. When you acquire interest in something or someone already fulfilling a goal, that fills a part of our ego as people and this allows us to get rid of the curiosity that has generated that interest and starts the demotivation to move forward to what is already known .

 People love challenges and obsess about having something unattainable for them, that is why they strive to achieve it and do not pay attention to the best choices that life presents to them because they will always have that uncertainty of wanting to have the attention of those who do not know about their existence for different reasons but with a mass consequence offering a single focus to that goal drawn by that person to feel the lack of motivation to care for other people.

 This can also be deduced as a social effect where most people seek the acceptance of groups of people of different tastes and with different routine activities that vary from those seeking to integrate into the unknown, with the social approach and the breadth of his circle of dealings with people, being very influential, always look for those human beings that are different bringing as effect a single focus seeking acceptance to the unknown and leaving aside the preferences of those people who are interested and approaching diminishing importance to its inclusion due to the lack of motivation that this produces towards the person.

 That is an endless circle where social and human maturity allows this routine to be cut and we value people for what they offer us positive and stay with those who really want to do us a good and not looking for other incessantly seeking its acceptance. 

Cause that was never a "true interest" in the first place. It was just based on wrong beliefs, expectation that in reality didn't turn out how they wanted to be

That string of actions resulted in them rewiring their brain, understanding how the interest they thought they posses, is not that at all

It as well can be related to EGO that's a pretty tricky thingy to handle. When EGO tends to chase someone (not real you) it tends to do everything in it's power, until it feels like you posses that person to a certain level

Afterwards, when it realizes how it has some kind of control sooner or later it will start acting totally uninterested, since as I said - the interest was never real in the first place

Rather than that the goals of ego are always something else - control, manipulate, get obeyed etc

So always watch out in reality, where is your interest in someone coming from? Is it based on their looks, or maybe on them not wanting to do anything with you? In both cases it's about your EGO

When something is real you'll know it from the start & be peaceful about it. In most cases it isn't as majority of us (humans :D) have a whole lot of inner work to do on ourselves. Before we can even recognize what "true interest" is!

I think it's all about attraction and having some things in common. When you've spent a considerable amount of time with someone and you discovered that such someone is not as exciting as you thought he/she would be, you'll easily lose interest in such a person and look for someone that provide the sort of excitement that you seek. Also, most people go after those that seem not to have interest in them because they like the chase or want to prove to themselves that they can get such a person.

People easily lose interest in those that they don't share a common interest with. Having an interest in someone is not a guarantee that they will also have an interest in you unless the attraction is mutual. You may be interested in someone and the someone may be interested in another. That's just how life is sometimes.