It really comes down to whether the fruit has a high amount of pectin naturally. Pectin is a carbohydrate that naturally occurs in fruit, but the amount varies depending on the fruit type and how ripe it is. Fruits like bananas and lemons are high in pectin and do not need it added. Soft fruits like berries and ripe apples are low in pectin so you usually need to add it to the jam to thicken it.