When you walk out of a theatre, often you can here someone grumble, “I could have written a better movie than that.” While that may be true (and often times the writer did actually write a better movie that just didn’t make it to the big screen for any number of reasons), I will say it is harder than it looks.
Just like professional athletes often make their sport look effortless, finding the right combination of characters and story and timing and rhythm is an art. I believe in hard work over “genes” though – that you can achieve anything you set your mind to. You don’t have to be born a good writer. It just takes A LOT of practice and patience.