A calorie is a unit of measure, and it’s relatively objective, when compared to say, how you feel about apples.
And it is important to have such units of measure, so that we can objectively measure, assess, and discuss certain things.
However, there is a bunch of grey subjectivity when it comes to calories.
So much so that you probably do not need to painstakingly agonize over how you count them.
If you want to learn more, read this.
Matt Fried 