The house edge, simply
Every casino game is mathematically built so that, over enough hands or spins, the casino keeps a small percentage of everything wagered — that's the house edge. It doesn't mean you can't win on any given night; it means that across thousands of players and millions of bets, the math reliably favours the house, which is how the buildings get built.
Why edges differ by game
Table games like blackjack (played with correct strategy) and craps offer some of the lowest house edges on the floor; slot machines and many side-bet options typically carry higher ones. This is also why 'strategy' matters more in some games than others — it can meaningfully shift the edge on games like blackjack, but does very little on a slot machine, which is purely chance.
Comps and free drinks aren't generosity
Free drinks while you play, discounted rooms, and other 'comps' are a direct function of how much and how long you're wagering — casinos track this closely through loyalty cards, and comps scale with your action, not your charm. They're a marketing cost built into the same math as the house edge, not a favour.
Treat it as entertainment spending
The most durable way to enjoy a casino floor is to treat whatever you bring as the cost of the evening's entertainment, set that number before you sit down, and stop when it's gone — the same way you'd budget for a concert ticket rather than an investment.