Because these foods are high in tryptophan and low in leucine and phenylalanine. (and contain cholesterol, fats and sugars when combined)
To know what foods can improve your sleep and happiness, you have to look at their SPF. ‘SPF’ = ‘Serotonine-Production Factor’ = tryptophan / (phenylalanine + leucine).
For example : if the SPF of a certain food-protein is 50%, this protein contains exactly as much tryptophan as the average of phenylalanine and leucine, strongly improving serotonine production. All foods with a SPF below 10%, more or less inhibit serotonine production. Whether these low-SPF foods actually do inhibit serotonine production, depends on how much protein they contain.
To enhance serotonine production, you should consume as little low-SPF foods, especially when high in protein. (avocado isn’t) So, consuming corn flakes for breakfast, bread for lunch, and pasta with lobster or horsemeat for dinner, is an extremely bad idea ;