RimWorld is a great game at heart, the gameplay is open and free to let you do what you want, becoming a humble farming colony, or making and selling human hats on the Rim's Black Market and slaughtering your enemies without mercy, the choice is yours, but, what makes the game addicting?
You could simply say the gameplay is just so enjoyable, it's hard to decide "enough is enough" and close it down for the night at 3am, tomorrow is another day after all, but I could say that the game makes you feel emotions. Happiness, sadness or anger, the game let's you build a sentimental, and valued connection to your colonists. I can't speak for everyone, as everyone is different, everyone conveys emotion different, however when I had my period of "wow I can't stop playing this game", I grew a connection with my colony that I spent over 100 hours building up to what I saw was perfection, and something I could say "I am happy with what I have built".
I was building an army, a strong one to conquer every last sentient human inhabitant on the planet, building my farms to produce more food then I could ever need, and decorating and detailing on every last section to make it my own. I loved this colony, and with that love came the slight sadness when colonists that I came to like, were brutally murdered by mechanoids, or passed in an uncontrollable fire that almost burned my colony back to the start. But why should I care? They're just pawns in a game?
They were the ones that built my colony to what I loved.
I came to posses favoritism for my personal pawn, he got the best armor, the best weapons, the new upgrades, and priority treatment when got a little scratch on his elbow. He would get the best decorated bedroom, and would lead my other colonists into battle. I would always be watching him, to see what he did, or if he got into any trouble, or if there was something wrong with him.
What I mean is that something you spend countless hours spending time with, you will eventually form a bond with, the bond being your colony, and your colonists. You will start the care when a little AI character gets married to another little AI character that have almost died countless times together, or when a little AI character passes away and you attend the funeral, with all the other little AI characters, or when your little AI characters defeat a horde of little enemy AI characters. That colony is like a pet to you, you take care of it, perfecting it, and eventually watching it fade away.
Call me crazy, but I believe that the thing that makes RimWorld hard to pull yourself away from, is the personal connection to the colony you hold. The blood, sweat and tears you accumulate to build something great, only to see it crushed to the ground like a sandcastle, and the emotion you hold with the event. It's sad, but joyful.
Thank you to Tynan, and everyone at Ludeon Studios for continuing to support this game for the longest of time, and here's to a positive and prosperous future for RimWorld <3
Thank you too! This was the goal of the design from the beginning, and I can barely believe it worked so well.