Why We Do It:
The main reason why I think we set monthly TBRs is for motivation. A lot of us have goals for reading. Whether we are participating in the Goodreads challenge, simply like to be consistent readers, or want to stay on schedule with our review output for the month, we usually need to have some way to motivate ourselves to stay on task.
Mood Reading - I know this is true for me, and I'm sure this is true for a lot of other people as well. Sometimes, we're just not in the mood for a particular book or genre. Or we're in the mood for a very specific type of book. For example, a few months ago I just really wanted to read a book in the vein of Rebel of the Sands. There were plenty of other books on my TBR, but something in my brain told me I had to read it. So I did. It's hard to explain where those feelings come from, but because of this inability to pinpoint it, I think it's all the more difficult to counteract these moods and actually read the books we initially planned on reading.
Life - This is obviously vague, but sometimes life just gets in the way. I know I'd rather be reading a lot of the time, but naturally life will continue to go on whether we chose to be living our own or living through someone else's (although those lives will pick up where we left off), so we sometimes don't always have the time to read as many books as we initially planned. My life always seems to get in the way. I know that I probably could make more time for reading, but I rarely ever meet my reading goals for a month (and I barely scrape by on my Goodreads yearly reading challenges).