The service a casino dealer provides is similar in some regards to that of the restaurant worker. They deliver (deal) the casino game to you. You expect this to be done in a friendly and efficient manner that will keep the game going without any unnecessary delays. Their other duties are to make change and pay your bets when you win. Unfortunately one of the dealer's duties includes taking your money when you lose.
In the movie, National Lampoon's Vegas Vacation, there are several hilarious scenes when Chevy Chase encounters the blackjack dealer from hell. This particular dealer takes great pleasure in beating Chase out of all his money every time he sits down at the table. He gloats and rubs it in whenever he wins a hand. It makes for some funny scenes but in reality the majority of the dealers want the players to win. The money you win is not coming out of the dealer's pockets so they would love to see you win. They know that most players who have a losing session are not going to be inclined to tip them.
It's human nature that when you are losing money at the table you are not feeling to benevolent. A good dealer will try to be friendly and personable so that the players will feel they are having a pleasurable experience even when they lose. Players having fun and enjoying themselves are more likely to leave a tip even after a bad session.
In some casinos the dealers gets to keep their own tips. In other establishments they have to pool the tips. I have found that dealers who keep their own tips tend to be even friendlier than the ones who pool their tips. I realize that this is a pretty general statement and I am speaking only from my personal experience. When I encounter a friendly and efficient dealer I will sometimes ask if they get to keep their own tips. If they do I tend to be a little more generous as I know they are receiving the full amount of the tip they earned.
In establishments where the tips are pooled, dealers will sometimes put pressure on each other to tow the line if they are bringing in their fair share of the tips. I had a friend who was a dealer in one of the smaller casinos in downtown Las Vegas. Where she worked the dealers on each shift pooled their tips. They had one dealer on the shift that was surly and very unfriendly and this was reflected in the tips from his table. The dealers on the shift had a "chat" with this individual telling him that his attitude was hurting all of them and he better clean up his act or perhaps go work elsewhere. His attitude improved greatly after talking with his co-workers.
There are several ways you can tip a dealer if you are inclined to do so.