If you write and want to continue writing, you can't do it to please the marketplace. You can't write a book about Vampires uncovering masonic secrets just because of Twilight and The DaVinci code unless you have a real passion for vampires and masonic secrets. If you want to write and continue, you can't write to please your best friend or your great aunt Martha. Indeed, if your want to continue to write, you have to write, first and foremost, for yourself. You have to write about the things that inspire you, that make you laugh, that infuse your passions. You have to, in other words, heed your inner voice.
If you want to better your writing you need to do more than just heed your inner voice. Writing is a two-way street after all. It is as much about the reader as it is the writer. Without the reader, writing becomes the proverbial tree falling in the forest. Language is about connection. The soul of writing is communication. To better your writing you must practice the art of communication.
You can of course write just for yourself. I journal regularly. I tend to journal around things that niggle at me in my personal life. I do it to help myself achieve clarity. The insights I get from this are often priceless. Though more often than not, my journal is one long kvetch session. Like a diary, this is writing that I have no intention of sharing.
I can journal until the cows come home, but this will not make me a better writer. To get better, I have to connect with readers. I have to find words that effect not only me,but others. I have to communicate.I have to move beyond myself and write with more than just myself in mind.