On some level, there's nothing easier than not writing a book. It's easy to come up with reasons.
1. I don't have the time, I'm far too busy. Look, I'm not going to tell you how to live your life. You make the choices about what you have to do, and what your priorities are. But if being a writer is a goal you have, you have to make it happen. Carve out the time. Choose it over X or Y or Z.
2. I'm exhausted, I can't write right now. I won't deny, I've had days were I didn't get it done. Hell, I had a period way-back-when (before I even started Thorn) when I had days where I couldn't be bothered to open up the file of my work-in-progress. But the only way to get it done is push through the bad days and be consistent and regular about doing the work.
3. What I'm writing is crap, I shouldn't bother. Like any skill, it takes work. Maybe it sucks, maybe it doesn't. But practice is the only way to get better. Lord knows I have plenty of stillborn manuscripts that never went anywhere, but that's part of the journey. Embrace it, give yourself permission to be bad and learn from it.
4. I don't know where I'm going and the story is stuck. All right, first: breathe. Take a step back. Work on a different project for a bit. Look at the outline and figure out what's the blockade. Skip ahead and write a scene far ahead, and worry about the connective tissue later.
5. There's no point-- you can't get an agent or a publisher to notice you unless you've already made a connection or know somebody. Not true. Flat out: not true. Whoever told you that is selling something (probably self-publishing services of some sort.) I had no contacts, no "in". I wrote, I queried, landed an agent after a lot of patience and then had more patience while my work say on editors' desks. And I wrote more books. I pushed through and did it because I devoted time and energy and patience to making that happen, with no "help" beyond the quality of my craftwork.
Write. Work. Push yourself. It can be done.