Because I love writing.
I used to write on diaries, but then, due to my so called "busy activities", I started to abandoned that; along with my comic making, novel writing, and cosplay hobbies.
Yet, I cant't deny my urge to write, which, then took form into updating fb status or tweeting. Again, it did not satisfy my writing urge.
I saw my little sis doing her blog and found it interesting; not to seek as many followers as you can ; rather it helps you to reflect on your own thoughts (and maybe sometimes to laugh at your own stupidities), especially if you are not too good or awkward in expressing yourself in front of many people.
Another point is, I think a lot, some people know me well said to me that I tend to overthink or overanalyze things; which sometimes drag me into unnecessary drama or dilemma in my life, so I guess, personally, blogging can help me channeling this kind of downside. A therapy, to be exact.