Pectin is what makes jam thicken and gel. It occurs naturally in fruit, although some fruits have such low pectin levels that you need to add extra pectin. Berries have lower levels of pectin, but still contain enough that the mixture will thicken on its own with the help of a lemon, a high pectin fruit. Jam with store-bought pectin can sometimes turn out too thick and have a rubbery texture. I think it also dulls the flavor of the fruit.