There are only two games in a casino where you can, in theory, make a profit in the long run, Blackjack and Poker. If you play perfect blackjack without counting, the house has a 1-2% edge on you. In the long run you will lose, but it's a great deal of fun and the ups and downs keep it interesting. If you can count cards you can turn that around and get a 2-3% edge on the house, but that's easier said than done. Try keeping track of a three person table surrounded by lights an distractions, all the while playing your cards correctly. I'm no slouch in math, but it's hard to keep that level of concentration for a three to four hour period that you need to make money.
Poker, on the other hand, is an interesting anomoly. You're not playing against the house, so your results are dependant upon your skill and the skill of others at the table. The house makes their money on the rake (a small percentage of the action) and rely upon keeping the game fair. Keep in mind, most people you see at a casino poker table have years and years of experience. They survive by waiting for the drunken idiots who can't sigure out the pot odds.
Even in poker, however, no amount of skill can keep a person from having a losing session. Let's take one of the less interesting games, Texas Hold 'Em (only because most people have seen it on television). If I was to play a person who showed his cards face up, I still would lose some hands. Here's an example. Let's say I was dealt 2 Aces. The person sitting next to me was dealt 7 and 3 off suit (one of the worst hands in poker). Obviously you would want to have all your money in the middle, but before the flop you only win about 85% of the time. Now, if I'm sitting down with a bad player I have a pretty good idea where I'm at. Most of the time, I have my money in the pot with less than an 85% chance to win. Even a fool wouldn't play the 7/3 offsuit, and obviously I don't get aces all that often. The best players int he world only win about 60-70% of the nights they play. Of course, any casino in the world would love to have a 10-20% edge in the games they run but they could never find people to play it. Poker players, however, can always find a new fish.
So why gamble if you can't win? Well, for one thing it's fun. Our brains are wired to enjoy the adreneline rush of putting something we care about on the line. Casino games are desgined to pay off enough to keep that fun factor going even when your wallet is being drained.
The saddest people in the casino arfe folks who figure they have a system. The slot junkies are painfull to watch. For the majority of people who go to a casino, they know in the back of their mind that they're going to lose, and the difference between a good session and a bad one is how much they lose. If they lose a small fraction, then they paid for the entertainment value and it's good. If they win it's fantastic. If they lose? Well, it depends on the amount.