The need to feel accepted by others is what drives people to post anything and everything on social media and the fun fact is that some of this may not even be true.
Another reason is the crowd mentality factor. If you do it and he does it, so should I - feeling which makes people obsessed with social media.
A few years before, I had a friend on social media who used to message me and request me to like her photos and posts. I did not see the reasoning behind this and although I obliged her once, did not do it again and again and she 'unfriended' me. Good riddance.
Even today, I notice many of my friends and relatives posting a lot of selfies and personal stuff on social media despite repeated warnings that the information could be misused. People just do not bother to think of the consequences especially when small children are involved.
The feeling of gaining publicity just like a celebrity is another factor which makes people post on social media.
The more likes you get, the more you feel accepted and so goes on life...