Ask this question to any of your blogger friends and they will share a variety of reasons - some do it for sharing opinions, for the love of writing, for money and then there are souls who blog just because everyone around them is blogging.
Finance blogger Ranjan asked Amit Varma the same question and his answer was "Guilt"
When I wake up late and log on to my site meter and see that a couple of thousand people have come to my blog while I've sleeping, it makes me feel guilty that I've let them down. So I blog on.That's so true. RSS is yet to go mainstream and a good number of your regular visitors will actually visit the blog website to read the new stories. Using Bloglines or Google Reader is just too geek and they prefer the old-school method of reading websites - type the URL in the address bar and hit go.
And when you build a good reader base who check your site every morning (or several times a day), it becomes your moral responsibility not to disappoint them. They are visiting your blog site with an expectation of finding new content but when there's none, some level of disappointment will be there.
That's where the Blogging pressure sets in.