I'll keep this brief. Why do you think we feel the need to share things with people? For example, you find a cool video and want to show it to others. We already enjoyed it, what makes sharing it more satisfying?

I guess, humans by nature are social creatures. We feel a need to share and spread knowledge openly, whether others accept it or not. Maybe we just feel that if we enjoy something, we might not be the only ones. Let others share the same happiness you feel. Stuff like that.

Validation

>>3 That too o:

>>4 Haha, yours was nice too.

To create conversation. Seriously, I can't come up with any topics that has nothing to do with the internet.

For reactions, sometimes when you enjoy it you think that person will enjoy it too. If its a serious case you might be looking for agreement or confirmations on the matter and if it's a scary video than you are just waiting for that person to jump at whatever pops out in the video XD that would be me hahahaha

I usually look at this one of two ways:
1) Histrionics
2) Need for socialization