For as long as man has had the means to memorialize themselves, they've taken every advantage of it.
Ancient peoples, separated by many thousands of miles and who could not conceive of the others even existing, left behind the most real and personal symbols of themselves their capacity for art allowed at the time - stencils of their own hands on cave walls. Such paintings were made in entirely different and distinct societies, tens of thousands of years ago, and have been found in France, Spain, Argentina, Indonesia, and many other places across the globe.
Many of the artists we recognize and appreciate as being the masters of their day - indeed, among the greatest artists to have ever lived - repeatedly used the tools that enabled expression, creation, and reproduction to present images of themselves in their works. Leonardo Da Vinci's self portrait in red chalk. The face of Nicodemus in The Deposition being Michelangelo's own. Caravaggio's David with the Head of Goliath, which is in fact the head of Caravaggio. When we think of Van Gogh, the image that likely comes to mind is the one he painted of himself and that is now in the Musée d'Orsay. We know what Rembrandt looked like through much of his life because he regularly painted himself throughout it. At the Prado in Madrid, see Velasquez himself in Las Meninas, and Goya himself in Charles IV of Spain and His Family.
Hippolyte Bayard and Robert Cornelius had to sit perfectly still for around 15 minutes for their early self portrait photos, taken almost as soon as the means to practically capture and actually retain a photograph came in to existence.
Now, I'm not saying your everyday selfie taker is a modern day Da Vinci, or shares a pedestal with the earliest pioneers of photography. As a species, however, we've gone out of our way to create and preserve images of ourselves by every means through which we've been able to do it - spitting dye over our hands to leave stencils on cave walls, carving ourselves in to ivory and stone, painting ourselves, recording our voices, photographing ourselves, and filming ourselves.
Even cameras have evolved to allow us to more easily do what we'd already decided we wanted to do with them. Timers came to be, presumably to allow the owner of the camera to also be a subject of the picture. Tripods let us set up pictures and videos we could then be in. The first camera phones and smartphones eventually had additional cameras built in to them, to make selfie-taking (and FaceTiming and Skyping) much easier.
We see so many selfies being taken simply because it's so easy to take them. Now, more than ever before, we're surrounded by people who have high-quality cameras at the ready and on them at all times. Further, each of those people are able to quickly and easily take as many pictures as they want, without going through endless rolls of film or the hassle of getting them all developed - point, click, done (maybe quickly and easily touch it up and edit it a bit for good measure). This selfie phenomenon we're seeing says less about any generation being self-obsessed or shameless, and more about technology - capable cameras on phones, and countless ways to share those pictures - enabling behavior we'd have seen decades ago had the means existed to engage in it. While I might be a bit too old or self-conscious to fully embrace participating in any selfie craze myself, I'm hardly patting myself on the back for simply being born at a time that made it something I had to get used to, rather than something I could take for granted.
As an aside, I suspect the friends that follow my Instagram are as weirded out by the endless stream of picture after picture after picture of my dogs sleeping as I am by their selfies! We're both just making the most of the technology at our disposal!