Why is the assumption of nothingness prior to existence so natural?

Actually there is no assumption of nothingness prior to existence. That neither makes any sense nor is it natural.

There is no "prior" to existence. Existence in some form or other means something exists. If you can define to me what 'prior' would mean when there is no existence then I might understand why you think that to be natural. At present your opening question begs the question how there can be a 'prior' to existence.

You could refer to prior to your personal existence, but that makes no sense. You know your parents were responsible for your coming into the world and you'd hardly consider them 'nothingness.'

If you mean the universe that we can observe around us, then I suggest to you that nothingness is quite the anathema in all we can see, measure and observe. Largely that is a result of the properties of that universe. If existence is greater than the universe we might consider there to be some valid concept of a 'prior' to our universe. However, that requires an assumption that goes beyond what we can currently examine and there is still an 'existence' that you'd need to dispense with in order to declare a time 'prior' to that existence.

At that point, it's turtles all the way down.

Labels are how we present an understanding. By defining nothing as the abscence of something, we give meaning to nothingness.

In our limited understanding we define existence as somehting of substance, something that is not lacking anything. From there our human logic brings us to the supposition that there has to be a before to anything that is something, yet logically something can't come from nothing.

This is the mystery or the great illusion. Our logic is telling us that there must have always been something for anything to exist. So what is this something that is nothing?

Some may determine that this is life. Life has always been and always will be, in some form or other. Life it's self (like nothing) is difficult to define other than to say that something that is alive has life. But what is it to be alive?

In essence the only reason things have any meaning are because we attach labels and put our own meaning on these labels to allow us to understand.

One plus one is two. This is not falsifiable. We can show this by reasonable argument. Is the concept of nothingness falsifiable?

Quite!

Also, are you asking from a 3rd person perspective or a 1st person perspective?

Just 10minutes ago I had an experience of nothingness. I was asleep. Between the dreams, I felt the same thing I must've felt before coming into existence and presumably the same feeling I'll have after I die - nothingness. Perceptual non-existence.

If you're talking about 3rd person description, I've never seen adequate reasoning for the existence of nothing. Where's any evidence of 'nothing' other than as an abstraction against which 'something' can be defined?

Is not natural. Nothing can't come from nothingness to existing. Everything exists eternally. Is matter of manifesting and unmanisted process.

Some species of Mayflies live for only 1 day. Because of this, they are also called day-flies. This insect don't see the night. Is this means the night doesn' exist?

We are like Mayflies, and because we can't see what was before 4 billions years ago, no even before we were born, we assume so many especulations according to our limited experiences.

Something and nothing are just 2 sides of the duality of life. Can darkness exist without light? Can good exist without evil? In all of duality, there is something that connects the 2 opposites. There is something that connects nothingness and existence... perhaps consciousness. But the existence of nothingness is not possible, for nothingness is a lack of existence.

Most humans practice a limited and limiting form of intellectual pursuit and understanding based on their misperception of lived experience.

They do not often or always choose to see that which is for that which it is without applying their own biased judgment.

The systematic and methodological method of rational thought used as a means of acquiring knowledge is not how they have practiced their personal consideration of the nature of existence.

Many still opine that pregnancy is some form of "magic" and mysterious occurrence even here in the supposed "modern west"....

Theologically, they like the sense of "magic" of their god as the agent of creation.

They also seek immediate self gratification in their own lives so that they want creation by a god to demonstrate the same again, like a magic act.

They seldom, if ever stop to consider how many years the magician trains to make the illusion of magic appear to not be an illusion at all.

There is also a fear of not knowing, appearing not to be in control and command so that to admit that one does not know is feared as a failure....

Interesting to note that the intellectual concept of nothing or "no thing" can not even been imagined as to think of "nothing" is to have a mental image of "something"...

I don't know, but it is annoying how many people make this assumption. They try to apply primate intuitions about time and space to the Big Bang, completely clueless about general relativity which was formulated a hundred years ago.

Because most people are not comfortable with other notions that are beyond normal perception. Are logic comes from our everyday experiences and is hardly absolute but patterns ingrained in us from evolution to cope with our immediate surroundings. Just like how Newtonian physics is only good for some velocities. Also like has already been said people bias. This may sound ironic to state but in my experience people believe before they form reasons to believe at times probably always.

Because humans cant grasp the concept of infinity. Were grown up on a '0' number system. I personally believe everything always was and is all the time.

1) In general, people should be contended with what is given to us by God and nature unasked.

2) OR It may be also called illusion or delusion, presumption, Inference or intuition or imagination. Or speculation through mental gymnastics. All these confusion has arisen only because "our mind was added to our body after we became existent."