In what sort of job are you most likely to have a relationship with a coworker?
If we discount showbiz and, to a lesser extent, the media, which are far and away the record-holders, some professions are notorious. In the medical profession, staff spend their working lives surrounded by illness and death, which brings coworkers closer together, helps forge close and intimate relations and helps them cope with the unique pressure their jobs puts them under. Any highly innovative company will also find its staff can easily get intimate, because if they're passionate about what they do it tends to to spill over into their personal relationships.
Why do so many relationships start in the workplace?
One, because you're under a lot of pressure and stress; and two, because the workplace has become increasingly depersonalized (we talk via email, phone etc, and have less human contact). Having close, even intimate, relationships is our way of restoring a more human element to working lives that revolve around Outlook and the phone.