Write because you must.
Write to show your unique take on the world and what matters to you.
Just as you’ve read stories and skipped the “boring” bits, make sure there aren’t any “boring” bits in your own work. Use your experience of what you like to improve your own writing.
Write to grip others (in a good way!).
Write true to (a) yourself and (b) your characters.
Why write unless you love doing so? It’s the only thing to get you through all the rejections. Try to see rejections as stepping stones to improving what you write.
To write is to live another life, to visit places you wouldn’t normally go to and, indeed, to invent your own worlds. To write is to put yourself in your characters’ heads and to discover many different perspectives (you’ve got to be able to have some sympathy with your characters, even the villains, to be able to write about them sympathetically). To write sympathetically means believing in your characters and to show them acting and speaking convincingly.