Your stories will reflect you
You might be forgiven for thinking this obvious, but it’s amazing/disconcerting how many writers are in denial about it. But we bring to writing all our life experiences --- good, bad, indifferent, all rolled into one big, messy ball of angst --- and it’s naïve to think we don’t write through that lens. For example, especially when you begin writing, your protagonist will most likely contain much of you --- hopes, fears, likes/dislikes. (Later, with experience, you’ll be able to write main characters markedly different from you, although don’t be surprised when little elements of yourself still creep in.)
Your stories are idealizations, usually not grounded in reality
Before spluttering in righteous indignation about how oh, no, your stories are grittily realistic takes on the human condition, shut up and allow me to clarify.
Take violence, for example. (Please.) The human organism is remarkably fragile in many ways --- one well-placed sucker punch to the back of the head can cause someone to drop with sufficient force that their cranium hits the unforgiving earth with enough force to instantly, as Will so poetically says, shuffle off this mortal coil. (Not to mention the damage done to the perpetrator’s knuckles.) But written stories --- and especially films! --- generally portray violence on a scale nothing short of cartoonish, with characters bestowing/receiving catastrophic levels of violence that put the Roadrunner’s treatment of Wile E. Coyote to shame. Without so much as a broken nail afterwards.
(If you stoutly maintain you avoid that --- because, you see, your stories are set in deserted cafés between worlds, two sedentary characters placidly debating life’s callous meaning or lack thereof… well, then, I salute you, Godot. But don’t ask for whom the Iceman Cometh. Because he cometh for you. And that right soon.)
Or fantasy worlds… which are generally heavily romanticized depictions of life in Ye Olde Dark Ages… you know, the era Thomas Hobbes so succinctly summarized as “nasty, brutish and short.” It’s all very well to wistfully/romantically expostulate about the Tower of Ecthelion gleaming like a spike of silver in the first rays of the morning sun, or to wander through the Wilds for weeks on end with nary a hair ruffled or out of place save when it maketh the character concerned fetchingly handsome/beautiful… but life just ain’t like that, folks. You try not brushing your teeth, or changing your clothes, or showering, for several weeks, and then observe the effects on you and those around you. (Actually, on second thought, don’t. Just. Don’t.)
Plan your stories all you want --- but, like “real life,” don’t expect things to work out exactly as envisioned.
I grandly call this The Myth of Control, blaming it on our warped culture, which continually, tirelessly, shamelessly promotes it. The myth? You Are In Control. Well, here’s today’s Life Lesson, folks: you’re not. You control almost nothing. Not even your own body. You can make choices --- in fact, that’s really the only thing you do control (don’t underestimate its worth!) --- but even there, don’t assume everything’s going to go as planned. Because it never does. (Are we talking about writing, or life? Yes.)
(Actually, at least as far as writing goes, this isn’t a bad thing. It means you’re not documenting actions of lifeless puppets, but chronicling lives of real people. And that makes a world of difference.)
Your writing will only improve as/when you do it regularly. Additionally, you must read.
This should be obvious, too, but many writers cling to the illusion they can write deathless prose without practice --- and without studying how the greats do it.
Well, let me tell you something else: the Muse can be a fickle bitch at times. She won’t show up if you don’t… sometimes, not even then. (Oy.) But she’s far more inclined to drop by, nonchalantly throwing a pearl or two your way, if you’re honing your craft than if you’re watching endless cat videos on YouTube or trolling Twitter.
To be any good, your motives must be pure.
Isaac Asimov said, ““I write for the same reason I breathe --- because if I didn't, I would die.” That’s why we do it: because we have to. Because we want to. Not because we’re writing the next Harry Potter or Lord of the Rings (although most of us wouldn’t mind that.) As in any field of endeavour, you’ll only be great if you’re doing it for the right reasons.
No need to be upset about these truisms (he said, ultimately throwing modesty to the winds). For example, I write fantasy… cheerfully admitting my world of Arrinor is heavily romanticized… and my protagonist absorbs/dishes out inordinate amounts of violence at times. (Only to bad people who deserve it, though.) And, yeah, he’s much like me in many ways… and sometimes, when I tell him to do something, he considers momentarily before replying, “Nope” and going off to Do His Own Thing, leaving me open-mouthed in the dust.
Just do what I do: acknowledge these observations as truisms… then don’t let them bother you, proceeding merrily on your way.