How many of you have ever watched a tearjerker movie and been sad afterwards? You get drawn in by the characters and their emotions, living their lives with them, laughing with their happiness, and then weeping as if you yourself were the one who lost your family and your pet in quick succession. It feels real, and so it becomes real to you. The visuals in the movie, the speech, and the actions all create a world that invites you to leave your own - in fact, it doesn't give you much of a choice. How many people saw Star Wars and spoke like Yoda for the rest of the day? Be affected by other styles, you will.
The same is true of books. What movies create with body language, props, and dialogue; a book creates with words. Think of your favorite book, and try to recall how you feel when reading it - do you look at the world through the same lens? Jane Austen, a classic, has a very individual way of writing. When I am in the middle of reading one of her books, anyone who speaks to me over that week or so will receive a Pride and Prejudice-esque response, in wording and manner if not content. If you want to tell me about your escapades in the latest bar spot at 4 AM, check what's laying by my bedside - its best to have that conversation with me when I'm reading something by David Sedaris rather than Mansfield Park.
What authors affect you? If you have a few minutes, try this exercise:
Pick a topic, say, going to the kitchen for a glass of milk when you can't sleep. Take this scenario, and try and write it in the style of several different authors - the quieter warmth with a bit of sarcasm like Austen, the clipped staccato of Hemingway.
Once you have tried a few different authors, try the most difficult one - write like yourself.
This will be a difficult exercise! Don't mimic the writer by using their words, but try and write as if you were indeed that writer. How do they differ from your own style?
If you aren't sure of your own style yet, then this exercise of impersonation will help you figure it out - what of those other writers do you see in your writer's voice? How do the writers differ from yourself?
Take a few moments and learn a bit about your own place as a writer, and remember: keep reading!