So here's what happens in the game, and really, in most shooters: you get a story segment explaining why you need to go through this level killing everything. Then you leave the dudes who you were just talking to behind, and go through the level killing everything until you get to the other end, where some more dudes meet you and point you to the next batch of things to kill. Some games give you and AI buddy or buddies. These buddies fall into two categories: cannon fodder that gets killed 10 seconds into the level, or a story-relevant character who follows you, sometimes assisting, and yelling at the enemy.
I've long since accepted that I really need storylines in my games. I can enjoy the gameplay, but I need a story to give me a reason to keep going when the game gets hard. Because of that shooters can be tricky because of the formula mentioned above. Generally, once you get into a level, the story stops. It's kind of like a Broadway show, where the story gets put on hold for a musical number, except shooters replace jazz hands with flying lead.
So what the hell does this have to do with Half Life 2? Last night I started playing the follow up, HL2: Episode 1. At least in the early going, you're accompanied by Alyx, a character that you ran into occasionally in HL2. What sets her apart from other AI buddies, besides the normal-sized, fully covered chest, is that she f-ing interacts with you. She comments on what's going on, asks how the hell you're going to get past this barrier, and generally behaves like a person. What winds up happening is that this interaction gives me the human element that I miss during the majority of shooter levels. While the story itself may not be advancing per se, at least I get someone to talk to.
I think it's the same thing that leads to so many buddy comedies in movies. If you were just watching Tom Hanks trying to solve a case by himself, it'd be boring. Give him a slobbering dog to interact with, and it's awesome! Hell, even Tom Hanks talking to a volleyball on a desert island managed to be entertaining for a while (though I could've done without the dentistry scene). Either Tom Hanks is simply enchanting to watch (possible), or movies/games are simply better when there's some kind of interaction going on.
For a perfect example of this, play Left 4 Dead in a room by yourself. To take this a step further, kill your AI buddies and don't rescue them. Now play it with four other live people. Which experience did you enjoy more? Same game, but I bet you liked the version where there were people. Proof positive that even something as inherently awesome as shotgunning zombies in the face is improved vastly by adding other people.