If you are serious about playing poker you have to make sure you are always in control of your mind. If your head isn't clear, your poker game won't be clear either and your game will probably get sloppy.
Poker is a game of psychological warfare comprising of a felt battlefield and mental ammunition. Have you got the mental fortitude to hang tough and beat your opponents with a sharp mind at the poker table? The up and down swings of no-limit hold'em can be severe and only the mentally strong players will be able to overcome a few setbacks.
You will probably remember that time when your opponent had the nuts 3, 4, 5 times in a row. To make matters worse, the same player was playing really aggressively in most pots prior to that, and you happen to have a strong hand, even though it was the second best hand. Because we have a selective attention, we will remember the rare occasions such as losing streaks. That's why those memories will come back easily when you are on another losing streak. But don't let that influence your play.
Let's say your opponent had the nuts 3 times in a row, and you always had a strong hand and paid him off or another player paid him off so you were able to see his big hand. The next hand you are dealt pocket kings and the player running good calls you. The flop consists of some low cards with no straight or flush possibilities and you decide to bet. Your opponent calls and you head to the turn. The turn is an ace. You decide to continue betting, because you put your opponent on a lowish pocket pair, which sounds reasonable. He calls and the river comes another ace. You decide to bet again and you get re-raised all in. Of course he can't have the Ace again can he? Suppose the pot before the re-raise was around $100, and you need $800 to make the call, should you really commit all that money?
Of course not. If you would never make that play then there is no reason to make it now. People think that coincidence always leads to variation. When flipping coins, people tend to think that after heads, the chance of flipping tails is bigger, when in fact the chances remain equal. And even though you are an educated and reasonable poker player, subconsciously this will influence your decisions if you let your emotions take control.
Keeping your mind sharp when playing is mandatory if you want to be a winning poker player. It will allow you to remain focused when involved in a hand, enabling you to play you're best game. It will also help you to identify if and when you start tilting off your chipstack, so that you can leave the game before you lose a significant amount of money.