At the beginning, I didn't even know it existed. Then I heard of someone's Livejournal and began to read their page every couple of days to see what happened.
After a while, I wanted an account of my own so that I could comment occasionally, so I asked that person for a code, and they gave me one (together with the advice to get a paid account since it has more benefits).
Initially, I didn't plan on using the account for posting anything into my own journal. Then I thought it might be interesting to write about how well I am doing and what's going on with my life -- mostly the more depressed sides that have to do with work. So I posted in that journal occasionally, and also commented in other people's journals.
After a while, I decided to open another LiveJournal (this one) to put my "normal" thoughts into it. I didn't think I'd use it much. My LJ-Sema client auto-logged in to the other account.
But as time went by, I began using this account more and more and the other account hardly ever -- now I post nearly every day and some days multiple times. I have a paid account here while the other account's paid time has lapsed. My client logs in to this account. I have several friends whom I read regularly and who read my journal.
I just thought it was interesting to go from not writing a journal at all (I have a book-form journal which, however, I hardly ever write in) to posting nearly every day, even though I hadn't planned on that initially.