Expert or newbie
But it should be remembered that there are different types of passion. For example, perhaps you are already an expert in some field or other and you want to educate others about it. (Many tech-related blogs are like this. They often include a lot of diagrams and step by step guides.)
On the other hand, you might be a complete newbie in the field you love so much. So you use your blog as a way of collating all the good information you find about it. Ultimately it becomes a kind of online course you give yourself and that others can learn from as well.
Another common blogging passion is politics. You could be an activist and you want to change people's minds about something. You want to inspire them with your vision for a better world.
Or maybe you've had it with all the utopian activism. You just you want to debunk it relentlessly. And a blog is certainly an effective way to do that.
Maybe you are interested in politics, but you're more of a philosopher. Rather than getting het up about current issues you remain more detached and analytical. Standing back form the fray, you use your blog to explain political machinations without taking sides.
You could also be a bit of an entertainer; a comedian. The main point of your posts is simply to amuse people rather than inspire, convert or inform them. So you use political developments, news and current affairs items merely as fodder for jokes and wry observations.
Perhaps your blog is motivated by a combination of two or more of the aims listed here -- or by something else entirely. Whatever the nature of your passion, it's worth really thinking about it before you start your blog, or at least fairly early on in its life. If you can clarify this purpose and stick to it then your blog will have cohesion. Readers who find it will immediately know what it is about and trying to achieve. It will be more compelling and shareable as a result, and it will likely become established far sooner than if you didn't have such a clear purpose in mind.