I had a realisation recently: "It’s all in your head” doesn’t necessarily mean “It’s not real”. This is a personal stance with feeling behind it, and my feeling is essentially that being told that something is “all in your head”, or suspecting the same, can feel for the most part invalidating, isolating and even a bit cruel. The specific example I had in mind was when people use this to explain when somebody experiences physiological symptoms which seem to have no obvious physical origin – typical examples being chronic pain, fatigue or fibromyalgia. In this case, “It’s all in your head” usually implies “There’s something wrong in your head”, which, true or not, may be interpreted as “There’s something wrong with YOU” “It’s all in your head”, in other words, to me usually suggests “It’s not real”, which is to invalidate someone’s personal experience – or at least to tell them they’re on their own.