Work is one of life's major priorities - we spend 220 days a year there, after all!
The office is like our second home; that's why we like to clutter it with photos, plants, clutter and a favorite mug, bring our playlist into the office, make friends with our workmates and go for drinks with them after a day at work. It's only natural. You have a lot in common with your workmates: the same boss from hell, the same pet hates, the same niggles and ideas; you work to the same goal and keep the same godforsaken inhuman working hours! You understand each other and you don't need to go into lengthy explanations of everything because they know instantly what you're getting at - which isn't always the case with partners!
Working together means forging deep affinities with each other, so if you work and sleep with someone it must mean you've a lot in common! When you work in the same company you have more in common than most in your day-to-day lives, and there's less risk of misunderstandings. You understand and respect each other's workload and responsibilities. Better still, you can really support each other. It's not as much of a problem if you have to stay in the office until the small hours to meet a deadline, get it perfect or find a solution to a problem, if you're both in the same boat. You're close to each other in every way.