So, Wikipedia says:
The psychologist Edward B. Titchener in his book A Textbook of Psychology (1928), wrote that déjà vu is caused by a person getting a brief glimpse of an object or situation prior to full conscious perception, resulting in a false sense of familiarity. The explanation that has mostly been accepted of déjà vu is not that it is an act of “precognition” or “prophecy”, but rather that it is an anomaly of memory, giving the false impression that an experience is “being recalled”.
What a load of bollocks. Here’s the real deal:
When you experience a déjà vu, it had already happened to you. Your experiences are fully controlled by your mind — you do not perceive some objective reality but your subjective one.
Subjective reality means you can experience different situations in a very similar way, when they put you in the same emotional state. More scientifically, the same partial chain of neurons in your brain fires in the same sequence. That’s why you remember it, that’s why you sense a déjà vu — it had already happened to you.
From the half-assed-guesses-I-am-absolutely-convinced-about department.