There's a constant mantra that SEO is all about unique content. So you are constantly trying to generate it. You keep worrying that you should be posting more of it as often as possible. Before long you start to run out of ideas, or at least you feel like you are. And you can simply become exhausted.
Needless to say, it's important to have a break every now and then. Of course it's worthwhile to stop blogging completely for a while. But it can also help a lot just to stop blogging and reading about one particular subject.
I've found that when I dump something I've been working too hard on and investigate something else instead I get a burst of energy. I also come up with a whole lot of ideas for blogs and websites related to it. Even though I don't begin building straight away, it's enjoyable just mulling over the possibilities. Then, after having had a bit of break from running projects, I do find it easier to return to them and generate new content.
Then if I start to burn out again, I can go back to those other, new project ideas occasionally. One or more of them will slowly start to gather momentum, and eventually become a reality.