Writing on Writing

Why is some writing so much harder to do than other?

This is a perfect question to ask myself as I am currently writing this text, which is not the text I should technically be dedicating all my time to (my thesis)...

OK, let's be fair. The most probable reason that comes to mind is motivation, or lack thereof. The thing is though, I feel quite motivated to write my thesis, because I want it done and I even have plenty of things I could write. Yet, somehow, I cannot but stare at my almost empty word document, unable to type in even a single sentence that would make sense. Why?
Furthermore, I don't feel particularly prompted to write this either, but somehow the word just keep flowing out of my brain, effortlessly.

This brings us to the second most likely reason, the amount of effort it takes to write a certain text. Writing a thesis is, without question, a tedious and time-consuming process. From time to time it even makes you feel as if a simple click on the delete button could offer you salvation from all your problems (I'm sure I'd regret it the exact same second I clicked the button though, if I was ever to go through with such an act; literal suicide, as my brother called it).
On the other hand, as I am an active story writer, and write something almost every day, background research and spell-checking should be something I am used to. As a matter of fact, I don't even dislike it that much. Plus, I do have an honest and keen interest in my thesis topic, so why is it so hard?

A third reason that pops into mind is distractions. Oh, there is a multitude of things I'd rather do than write this one assignment. Play video games, work on my own stories, watch movies, take a walk, draw, you name it! I'd do just about anything else and yet, simultaneously, I want to write my thesis and wish I had the same drive to do it, but I don't... Why though?
I have even gone to such an extent where I have promised myself not to do any of the things I want, not until my thesis is finished. And, to most part I have actually kept this promise, but still, I make no progress... It's quite horrible really...

A fourth, and final, reason I can think of is the fact that I am "forced" to write the thesis. However, ultimately, it's really all in my hands. No one is actually making me write my thesis; I do it because I want to graduate. I do it for my own sake, in a similar manner to all my other writing, so why is it only the thesis that causes me such problems???

Maybe this is just a phase everyone has to go through when working on a major project...
I don't know, but I wish it will become clearer with time.... I really hope it will...