I'm a single player person, I love a game for the story, I don't mind playing co op occasionally but for the most part I like to lay back on the couch and get lost in a great story (pity I got mass effect on PC rather than PS3 so have to hide in computer room to play that).
Now as for why people are getting upset, there are so many Multiplayer games already, even where the games SHOULD be single player they have been pushed into multiplayer and lots of us are just not that interested in playing multiplayer
I can see exactly why the wolfinstin Devs have gone SP only, thats the history of the game, its not COD where its all about the competitive MP and the chances of drawing that crowd in are low so dumping that to focus more on the story and the experience for those of us who play SP is fantastic. I'm not against optional local co-op games either if they can pull it off so it doesn't hurt the SP experience (unfortunately that means games like resident evil 5 where the computer player SUCKS). In reality a good example of this is probably diablo 3, you can play single player and if someone joins you the difficulty goes up, now if they could do that with a console split screen shooter or RPG that would be great but if its going to hurt the SP then don't bother
I have no vested interest in Titanfall because its on the Bone so I haven't even looked at it but I would say that any disappointment would come back to how it was announced, was it announced as a MP game only or was the story pushed the way companies market SP games and then days latter they come out and say "oh BTW its only MP"? No one would go into wolfinstne ONLY for the MP, they MIGHT chose to play it multiplayer down the track but its bought as a SP game and its fans are SP fans. Titanfall is new IP so people are looking at it based only on what was announced.
I will say though that if Ubisoft came out now and said "oh BTW, watchdogs is MP only" people would be very peeved (including me) because that's not the expectations that we have for that game based on what has already been stated