For me it was definitely a case of falling into it. As a Sys Admin, myself and a couple of colleagues had to learn several technologies for a project at work, so we split them between us and I ended up with SQL Server. I think the tenuous reason was that as the person responsible for backups I'd spent time in SQL getting the backups to work, and therefore knew slightly more about it than the others.

I went on to do my 2000 MCDBA, and ever since it seems I've been "the DBA".

I'd still describe myself as a Sysadmin rather than a DBA, since SQL is only part of my job, but I definitely enjoy it. I think what attracts me to it is that many of the parallels between Sysadmin work and DBA work are exactly those things I enjoy most in my job.

I've certainly thought about focusing more on SQL in the future, but aside from that definitely requiring a job change, I'd miss playing with all the other tech!