Not an easy question to answer, other than the superficial copout of "I just like playing." likewise, I'm sure there is multiple reasons why we all play--some big, others not so much.
One of the big reasons I play is that I love playing with my clan. We're mostly US Air Force and used to be stationed together and this is the main way for us to continue to keep in contact. We also win way more than we lose and that helps too
One of the smaller reasons I like to play is I love the animation of when I catch someone with a shotgun. It's comical to watch them fly back.
There are other reasons, these are just a couple of mine. What keeps you coming back to this game?
Competition! We all want to win be it singularly or clan. There is the social side but I find you don't seem to be able to have the craic on ghosts as before. But overall it's FUN! Build a soldier, kit him out and see how it goes. Ghosts beats blops2 in so many respects but for me using a thing called a brain gives me more satisfaction that even though I've got a number of challenges on blops2 that are 90% I've not taken ghosts out of the machine.