What it all comes down to in the end is that reality is subjective.
If you haven't come across that term used like that, let me explain a
little.

There are two modes of perception, subjective and objective.
Subjective perception relies on the input of the senses and is experiential in nature, that is, it exists because we percieve it to exist.
Objective perception relies on proveable fact. The thing percieved is true no matter what we think about it.
However, the problem with objective reality is that it is very, very limited because there is only one thing anyone can be sure of beyond doubt and without relying on the senses in some way. That one thing is one's own existance. Rene Descartes, the philosopher, summed up this surety of self awareness in the famous quote "I think, therefore I am".

You cannot prove beyond doubt that any thing beyond that,
including the existance of anyone or anything at all other than
yourself, is not imaginary. You can't even prove that you are a human
being rather than a brain in a jar or even pure energy floating in
space. You could be imagining any or all of that. All you can be sure
of is that you are thinking, and therefore you must exist. And you
certainly can't prove you existance to anyone else. They might not be
real, or to them, you could be a delusion.
   So that leaves subjective reality. In this mode, we assume that
most of what our senses tell us is real, and we form opinions of
those things that we percieve based on our previous experiences with
other percieved things. Because our senses are so heavily involved in
the constuction and definition of our personal reality, the world we
live in is largely based on our opinions.

Now, even twins have different experiences from each other during their lives. No two people will ever have the exact same background. Because of this, the subjective reality of any two people will never be exactly the same.
This is the "where you're coming from" that you sometimes hear people
say they understand or don't understand.

For example: Both Sue and Jane had Cocker Spaniels when they were growing up. Both dogs were typical of the breed. But Sue's developed a kidney disorder and died while Jane's lived out its life and died of old age. In Sue's world, perhaps, Cocker Spaniels are more likely to get sick than other dogs. She might not logically or rationally believe this, but if she were getting a dog at the shelter for her own daughterm she might pass by a Cocker Spaniel and get a Beagle instead. Jane, on the other hand, would likely think of Cocker Spaniels as healthy, happy, long-lived dogs and would choose the Cocker Spaniel at the shelter over any other dog because of her good experience with the breed.

Both of these are subjective opinions. The objective fact is that Cocker
Spaniels are about as healthy as any other breed, no more and no
less.
   Our relationships with people are much more subtle and complex
than choosing a dog. We make instinctive decisions about people based
on all the experiences we have had with people since we were born,
most of which we are not conscious of on a daily basis. No two people
will have had the same experiences, so no two people will have
exactly the same opinion about someone.
   This is not to say that we cannot reach a concensus, that is, an
agreement about the way things are in general.
For example, the popular concensus is that on a clear day, the sky is blue. We can all agree that it does appear to be blue, so we say that in fact the sky
is blue. We can't prove it. It is a subjective opinion. We can't even prove that there is a sky. We could use scientific measurements, but all we can do is agree that the instruments we use seem to exist, and that the readings they give seem to mean something in relation to our previous thoughts. And the concensus breaks down once we get past the very surface of it, because no two people will have exactly the same idea about what "blue" is.  
To prove this, get a bunch of paint sample cards at the local home improvement store that have all the different shades of blue on them, then ask your friends one at a time, (so there is no concensus-building going on) to choose which shade is the "most blue". (Make sure to white out the color names so that there is no suggestion about a "right" answer, like a color
called "True Blue".)
What you will find is that there are differences in their perceptions of what "blue" is. This might be because they remember on a subconscious level what the hue of blue was in a "baby book" that was used to teach them colors. Or their father might have remarked how blue their neighbor's car was when they wer a toddler and that idea stuck, or any of millions of possibilities. And we haven't even touched what blue means to them.

Again, a concensus has been reached that blue is a sad color. But I like blue. To me it is happy. My first car was blue, and I loved that car.
   Every day we live in a society that is guided by concensual reality. This is the (tacitly) agreed-upon basis of what is real and what is not. As a Witch, you will be living out-of-step with concensual reality. To the average person, there is no such thing as faeries, nature spirits or magic spells outside of fantasy stories. A Witch consciously creates his/her subjective reality. In fact, most of magic is using the Will to effect chages in subjective reality.   The "weight" of concensual reality keeps magick confined in most, (but not all) cases to the subjective reality of those directly involved.
   I hope this helps.
It's really a super-condensed treatment of a really big subject. There are fine points missing, but I hope it gives you a general background of why people see things differently.
Blessed Be, -Dragon