I think it has to do with peoples want to portray happiness rather than the actual experience of real happiness. It comes off as fake.
I could bore people to tears with videos of me fly fishing when I am perfectly happy and at ease with my surroundings, but what's the use? It really isn't very interesting to anybody but me in that moment, and setting up a cam or trying to hold a phone would ruin it. But what people really seem to like to do and see is set up a scenario to take a selfie with a big daffy smile, when that isn't how things go down in real life. Then I'd see the same selfie people at the grocery store or in passing and they terminally look like someone just kicked their dog, and all they want to talk about is what so-and-so said on facebook.
I've seen some neat stuff on social media too though. It's great for special interest groups, and I also participate on a welding forum, but they don't seem nearly as prone to the superficial shows of emotional investment and happiness that some platforms seem to be.