Tripwire Interactive’s Killing Floor isn’t a mainstream first-person shooter. It’s not even that popular with hardcore gamers. That doesn’t mean, however, that Killing Floor isn’t one of the best designed multiplayer shooters out there today.

The game has its flaws—it looks a bit ugly and is really buggy—but I still play it virtually every day. I found myself wondering why that is, and this article will aim to explore what makes a game so moreish.

I think the main reason why I find Killing Floor such a pull is a very simple one, and probably something a lot of people can relate to when playing their own favourite multiplayer game. Put simply, my freinds play it too. That’s the single most important thing about getting players to switch on as often as they can. Games can be incredibly social things, and if you can manage to get groups of mates playing them then they will keep coming back.

It sort of stretches into the realm of psychology. Most people have heard of Pavlov’s Dog– the experiment where a dog was given food when a bell sounded. Doing this enough times led the dog to associate the bell with the food, and would salivate to the sound of a bell even when no food was present.

It’s not the same thing, but the principle is similar. Start playing games with friends and you will enjoy yourself—probably mainly because of the social aspect, though having a fun game will obviously help. As you keep playing, you begin to associate the enjoyment from socialising with the game. I think that’s a big reason why I love Killing Floor—it has good feelings attached to it in my brain.

I don’t think it’s just that, however. Killing Floor also has the second most important thing going for it in keeping a player’s attention: a compulsion-forming mechanic. Like Call of Duty, there are perks and upgrades to be had just by playing the game. I actually think Call of Duty does this slightly better, with much more to unlock spaced more closely together, but the system is still there.

Again, this is linked to something the human mind does in the background. Every time you level up in Call of Duty, it dumps a pile of endorphins onto you- the chemical associated with a feeling of well-being and happiness. So you feel good for a bit. Then, of course, you see the next target and you want to keep playing to get that hit again. It’s basically like being a heroin addict, only slightly more dangerous.

There’s a reason why I think the leveling up system is so great in Killing Floor, and in a different way to Call of Duty. The perk rank-ups are so spaced out that they only really happen once in a blue moon, but they are really significant. I won’t list them here but the fact is that they make a big difference to the game, and the drive to get to the next level is really compulsive. Importantly, the endorphins needed to keep the game compulsive are ever-present in the gameplay. Killing Floor is a co-op survival shooter (Horde Mode, essentially), meaning that you’re not killing fellow players but bots. Usually this doesn’t become massively addictive- it’s not a massive pull in Left 4 Dead, for example (though that game is great for other reasons)- but the developers of Killing Floor really got it right.

Firstly, the game is hard. Really hard, especially on the upper difficulty levels. Obviously there’s a steep learning curve, but I think if you’ve got the “playing with mates” and “compulsive leveling” aspects down, I think even the worst players will see past that. The importance of this is that every enemy is a struggle to kill. The weakest enemies in the game, Crawlers, don’t have a lot of health but instead are low down, out of sight. Other enemies are invisible. Some are just bullet sponges.

This means that every time you get a kill, you get some endorphins (remember those? good). Oh, and did I mention that in a “long” game (the one all the servers are set to), you get about 200 of them to kill, every round. For ten rounds- so you’re getting hit after hit of the happy juice. This is also helped by the game having the best, most satisfying headshot noise in all of gaming (crack, as the head explodes).

Killing Floor isn’t the best game in the world. What it does right, however, is exactly what brings us back to games like it. With the social aspect, leveling compulsion and the steady stream of endorphins from each headshot, Killing Floor continues to bring me back to it nearly every day. Of course, this is just a personal thing. I’m sure we all have experiences like this, most with different games to me—but I’ll bet that the root of your enjoyment will be the same concepts.