This is a huge question.

And, honestly, it's not one I'm really qualified to answer.  If you're looking for opinions on this, Susan Sontag's On Photography is an amazing book, though purposefully contentious, and while it's arguably not aging particularly well (her argument about photography being voyeuristic, or the camera as barrier to the world around us could not possibly survive the unforeseen coming of Instagram, Snapchat and the proliferation of digital images) it's still fundamentally relevant.

But what I would like to do is vastly over-simplify the issue down to two motivations; do we take photographs for others, or we take photographs for ourselves?

Taking Photos for Others
The drive to share an experience is noble and identifiable. It's why we take holiday snapshots, or take photos at a wedding. The ability to document big life events was enhanced greatly with the invention of photography, and once digital photography became commonplace and ubiquitous there was no longer any reason not to document the little life events, too. Selfies aren't remotely new, but they've never been easier. Taking a shot of a nice-looking plate of food at a restaurant might not have been worth the cost of film and processing (and the hours/days you'd have to wait to get the photograph back), but it only takes a couple of seconds now to have people all over the world clicking LIKE or LOVE. It's almost like they're there with you.

Technology is obviously behind all of this. A high-quality, simple-to-use digital camera is in the pocket or purse of about half the world's population.

A guy called Ben Evans did some research in 2015 (already two years out of date, so all these numbers are going to be even higher now). His estimate was that the total number of photographs ever taken on film ranged between 2.5 to 3.5 trillion. That's over a period of, say, 180+ years.

According to the report InfoTrends Worldwide Consumer Photos Captured and Stored, 2013 – 2017, 1.2 trillion photos will be taken in 2017. That's nearly double the number from 2013.

Let's assume that almost all of these images are going to be shared. Social media is highly optimized around sharing images, so let's also assume that the intent when taking the photograph was always going to be sharing it with others. 

There's immediate feedback when you post a photo on social media. Likes, shares, comments; these are all little instant psychological rewards for taking and sharing a photograph, and the more encouragement one gets, the more photographs one is likely to post.

It's a feedback loop encouraging the taking and sharing of more images and that's why these immense numbers are steadily increasing year-on-year. Instant gratification on multiple levels: you take the shot, you immediately see the image, you immediately share the image, you immediately get encouragement.

In the majority of cases, I'd argue that the vast majority of modern photography occurs after the thought, "I am going to share this."

This is what I mean by taking photographs for others. What we get from it is recognition and affirmation from our social media connections, in those immediate little brain chemical kicks that we get when we get a like or share. 

Sometimes I'll see something that makes me reach for my camera. Usually I am thinking, "what would x think of this shot?"

This imaginary audience is almost always a specific person, or a handful of people. Much less often, I have a generic audience in mind, and I'm thinking about if the shot is commercial or not; could I sell a prints of this? I'm rarely this calculating, though. That said...