Part of it, I suspect, is ego. Not that I'm proud of that, but it's a factor that demands consideration.
Another, arguably healthier, part is the writer's need to write. I'm usually not that talkative in person, but sometimes I think my head will explode from all the "useless information" (my wife's term) piling up in there. A blog may help me exercise my brain while exorcizing some of the data.
If, somewhere down the road, you want to blame someone for the existence of this blog, look to the guys over at Querencia. Steve happens to be one of my favorite writers; I can't even remember now how I discovered his blog, but I've become addicted and will look to it as a model of what a blog can and should be. Matt actually sowed the seeds by suggesting that I blog on my first dachshund, Maxine, when she was a puppy. I wasn't ready then, although I did eventually write an article about her and two friends' rabbit dogs for International Falconer magazine. Now I think I'm ready to take the plunge, although I'll try to make the subject matter slightly broader than training one puppy.
Here goes nothing...