"Happyness comes to whom belives in it." (sic)
I didn't grow up with any exposure to graffiti or urban artwork, I did not understand it until much later in life.
Nowadays, urban art catches my eye once in a while. This one I spotted in Montréal downtown -- an urban space in itself that is vibrant with life and colour.
My own belief as to why someone paints a wall like this, is the blandness of modern architecture. Look around, and you'll notice more often than not that "vandalism" tends to happen on textureless, plain walls. Graffiti, tagging, etc doesn't always necessarily improve the look, but certainly looking at the bland numbness of modern design I can apreciate why an aspiring urban artist would decide to choose the locations they do.
A play grey concrete wall looks too much like a canvas to resist, for some.