The game is seemingly designed to make the players hate each other and encourages them to screw each other over. The object of the game is to get stars. Obtaining one takes a considerable amount of effort. From any point on the board it can take several turns to reach wherever it is located. Keep in mind that you also have to beat everyone else there. Even if you're first, you have to have enough coins to purchase it.
Getting to the point where you're about to get the star takes some luck and time, but even if you're one space away, so many things can go wrong.
You can lose your coins through some weird game event (this happens all the time) and no longer be able to afford buying it. If your own mother has the opportunity to pick a player to steal coins from during that turn, you can be goddamned certain that she is going to take them from you.
Your friend who is way farther back than you can roll and improbably high number and pass you to take it for himself.
Someone can actually move the fucking star to other side of the board before you can get it.
Keep in mind that any of these occurrences essentially means that all of your luck and effort over the past 3 or 4 turns essentially amounts to nothing.
Worse still, even if you get lucky enough to actually beat everyone else, keep enough coins, and get the star (usually 4 turns worth of effort), your friend can steal it later in the game. The worst part about this isn't that it's random, they have to specifically choose you to be the player whose star they will take. I have seen no small measure of vitriol exchanged between two normally level-headed best friends over this. Sneering, shouting all manner of obscenities at each other with no hint of irony, walking out of the room, etc etc.
And don't forget about the fucking bonus stars. You could have a cushy two star lead on everyone else by the end of the game, but guess what? Second place gets all three bonus stars, which are awarded based on certain criteria at the very end of the game. Things like landing on the most green spaces, winning the most minigames, or fucking buying the most things at the shop. You can't know which of these qualifications will matter because they are randomly chosen by the game.
Basically, it takes a lot of luck, time, and effort to get to the point where you're winning, but one single event, sometimes intentionally targeting you, sometimes utter chance, can completely ruin that and relegate you to last place. Now imagine this happening over and over again over the course of two hours and you'll understand why playing Mario Party will make you hate your closest friends.