who painted my door?

this is a photo i took, in a sidestreet in Barcelona - i love the colours, the artwork...and the spontaneity of it. the old man had no idea he was having his picture taken. my favourite sort of photography. barcelona is great for wandering round, looking at graffiti - there are some really creative artists at work - and of course, the ubiquitous tags scrawled on available empty spaces. i don't personally understand the mentality of tagging - especially when there are 3 tags within about 4 feet of each other. why would you do that? do you have the memory of a goldfish?