If you know others and know yourself, you will not be imperiled in a
hundred battles; if you do not know others but know yourself, you win
one and lose one; if you do not know others and do not know yourself,
you will be imperiled in every single battle. - Sun Tzu
Chess is a game with countless variations. It is commonly known that there are over 9 million possible positions after just three moves. Nobody is expected to explore all of them, not even the mighty supercomputers. There will be lines which are simply not fully explored in every game we play.
One of the early algorithms you learn when developing chess computers is min/max. It's a rather blatantly obvious algorithm when you get down to it: the idea is that both players are trying to maximize their own benefit. In min/max, we treat this as you trying to maximize your advantage, and they are trying to minimize it.
If you play chess using the kind of thinking behind min/max, you quickly find yourself limited. No two players are thinking exactly the same. They value different kinds of positions, different tactics. You may find that it isn't effective to model your opponent as trying to minimize your advantage. You have to recognize that they are merely trying to maximize their advantage. The two phrases, "minimizing your advantage" vs "maximizing their advantage" are closely related in many situations, but they differ greatly in others. In fact, Chess often hovers in that meddlesome region where the position is so strange that both players are approaching it quite differently. Nowhere is this more visible than in a gambit. In a gambit, you offer something in exchange for an advantage that you find more valuable. They don't have to take it. However, in many situations they may decide that it is in their interests to accept the gambit. Min/max cannot explain a gambit; it cannot explain why both players would want the same position for different reasons.
Accordingly, it is essential in Chess to get into the head of your opponent. Grandmasters may do this for months, studying up on the play styles of their opponents.
An opponent can get into your head at any time. Kasparaov v. Deep Blue may be the most famous example. In one game, Deep Blue simply refused to accept an obvious materialistic move and found a more "human" move. For the rest of the tournament Kasparov was convinced there had to be a human helping Deep Blue out, and that psychologically destroyed him (even though the computer wasn't even aware of a psyche).