One is because you feel like eating and the second is that you have to, or you will literally die if you do not. Let me put it another way. Do you want to eat or are you kidding yourself into thinking you should be eating because its a tasty thought in your mind?
Let me ask you a question and this applies to many subjects. Do you want to write or do you have to write to release the demons that keep telling you, this is what you were meant to do.?
Writing is a passion that grows and grows with each word you write, whether your subject is fact or fiction, about others or about yourself. The first word is always the hardest and it rambles on from there. It does not have to make sense, you will have time to correct that later.
A burning passion to write will drive you forward until you master the technique of either telling a story or maybe describing in picturesque detail how to bake a cake. It is not as simple as putting pen to paper, writing is a skilled craft and needs study and discipline to succeed.
When writing a page of text you could end up editing over half of the words out before you are finally satisfied. And then when someone else reads what is left, you could be in for a shock. What you thought was brilliant piece of the written word will be rejected for many reasons you do not agreed with. Your pride will take a fall and you will have to recover from this, or face never getting into print.
You have to give other people credit for seeing your piece of writing for what it is, especially if they are professionals i.e.: authors journalists or editors. Many new writers do not take criticism kindly and instead of taking these peoples comments on board and using them to improve their work they consider it as an attack on themselves.
We all like to think we know best, but writers need all the help they can get to move forward and keep improving. Writing a story is about painting a picture in words and the skills to do this need to be sharpened and honed until it becomes second nature to paint a compelling story that anyone wants to read.
It takes discipline and drive to keep writing and improving. Write about your thoughts, your anger, your pet hates virtually anything where emotions are involved. Read it out aloud and see if it makes any kind of sense. Print it off and read it aloud again. Leave it a week and then go back and read it again. Do not ask family or friends to read it they will be too kind.
Try this simple exercise: Get your partner or friend to close their eyes then place a piece of fruit or other object on a table and now try and describe the object to your friend using your powers of fiction without actually telling them what the object is. Then ask them what they thought about your description and what was lacking when they discover the answer. You may be surprised.