Great advice - Art or graffiti?

I saw some graffiti on the wall of a side street roday and I got to thinking;

When does graffiti become art? 
What is art?

Is it when the creator has a motive or a reason behind the work?
Is it when the audience viewing the work likes what they see?

People are continually challenging the boundries of art. 
We only have to look at the work of controversial conceptual artist Damien Hirst to see examples of people (artists?) challenging the concept of art by using sometimes shocking methods.

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Some may say that his more intense work is absurd. 

Others may view it as inspirational and misunderstood.

Do 'artists' feel the need to challenge art or is the work they create something they believe in?
Or does this vary artist to artist?

When we look at expressionist artist Jackson Pollock, in my opinion there are signs of an art challenger!
Pollock created paintings in a way that people had never really seen before. 
Was he an innovator or an art challenger?

Is something only art if the audience has a chance to make an interpretation?

Should we stop looking for answers or is that just human nature and something we will never allow ourselves to do?

If I stand in a museum and switch a light on and off while people are watching, would this be art?
If the answer is 'yes' then is it still art if I do the same thing in my own home?
Is everything in the confines of the museum walls classed as art? 
Should we be told that something is art?

Surely we should all decide for ourselves what we feel is artistic and not be told what to think?

Does art have to be something we enjoy looking at or experiencing or can it be art even if we don't like it?
Is art only art if it evokes an emotion, be it positive or even negative?
Does art have to make the viewer feel something for it to become art?

Science and knowledge can proove certain things, but the question of art cannot be so easily defined.

So many questions and I don't think there will ever be an answer.

In summery, in my opinion...

Art is whatever you want it to be regardless of it's confines and it's beauty or lack of.

So here is the art/graffiti that prompted my many questions...

The creator / artist wanted to send a message. 
This wasn't just mindless scribbling. 
The artist had a stencil and thought about what he / she was doing before putting ink to wall. 
It wasn't something that they did with no motive or thought.

I wish I could find out who put this message on that wall, or maybe the mystery is the interesting thing!

Why this message?  Why this wall?

I think it's especially interesting the way such encouraging and positive words are painted in such a dark and contradictory fashion.

Do you think this is art?

Do you have answers to my otherwise un answered questions above?