Top ten, more or less, reasons for writers' block:
1) What you're writing is a lie. It just ain't true. You're bullshitting, and on some level you know it. There's an inner critic inside your head going "this is shit...what total shit" and, positive thinking, visualization and warm fuzzies aside: sometimes that little guy with the pitchfork is right.
2) You're not the same person you were when you started the story. Hey, ohmygawd, guess what: you're not a snotty adolescent anymore and aren't all that motivated to finish your whiny little buildungsroman (with its whiny, suffering, little adolescent hero). Fact is...you're really starting to hate that guy.
3) Stagefright. If you finish it, somebody might print (or produce it) and a real audience might actually see it....judge it...might not like it...
4) You're more into the role of being a writer than actually writing. This could take one or two forms: you go around telling everybody you're a writer and they believe you because you're clever, glib, and use big words. I.e.; you've already had your payoff.
Or...you're putting all your loved ones through a guilt trip because you can't write. "If only you supported me! If only you loved me...but you don't! That's why I can't get this thing done. I don't get any $%#^ support around here." Your payoff is the guilt points, not the finished work.
5) You're lazy. Nobody talks about construction workers' block...but there you are, you've told everybody "should be done in a couple of months," but it's hard...you're nowhere near being finished...you've had to change things...it goes on and on, your wrists hurt (carpal tunnel syndrome!!!), your butt hurts, your brain hurts, and you just didn't know it was going to be so much work.
6) You aren't making any money at this and (a) something inside you says there ain't no cheese down that particular maze (b) a significant other is wondering how you can park your ass in front of the typewriter with all the bills to pay, all the work there is to do...etc...
7) You're a perfectionist. Every word, every sentence, has to be perfect before you can squeeze it out of your brain onto the page.
8) You don't know what you're doing. Little things like dialog, plot construction, characterization...
9) The most life experience you've had is working retail at Burdines one summer. As in: you've never done anything, gone anywhere, had any fights, had much sex, gotten drunk, taken drugs, volunteered for anything, voted, been in danger, talked to anybody outside of your narrow group, if that; have been living inside the narrow sphere of your own head; don't have much to say because you've been avoiding...gahhhh...that thing called life.
10) Your own imagination is working against you. Getting this thing done is so real to you in your mind that you think you've already done it and are already bored with it.
And...one to grow on....
...you're spending all your time getting drunk, high or having sex...and there's no time to write!!!