Apart from the fact that he seemed like a thoroughly nice bloke, I think one of the reasons he's so mourned is that, for many of us, we've never not had David Bowie. People born as far back as the 1950s, (and therefore teenagers in the 60s when he started) have always had a Bowie song or two kicking around in their life. He's been present on the music scene in every decade since the 60s, never losing his relevance and always coming out with something new and ground-breaking.
Bowie was eccentrically arty but talked like a normal person. When you see him on talk shows and other footage, he's not only got an unpretentious accent, but he doesn't spout high fallutin' stuff and never strives to make himself out as an intellectual or an artiste.
The fantastic 2013 Bowie exhibition that was sold out for months in London, came to Chicago in 2014 and I was lucky enough to see it. Alas, since he didn't much care for his more disco-ey songs of the 1980s, there were virtually no references to that decade which was a shame for me since I saw his three tours - 1983, 1986 and 1990.
In fact, here's my own piece of Bowie memorabilia - me at the Milton Keynes bowl in 1983, at his "Serious Moonlight" concert.
Yes, I know it could be anywhere, but I know where I was!