So he's easy to talk to when you're among the last to leave the office at night? OK, but don't fall into the trap of building a relationship around your professional lives. Everyone has their own personality, interests and habits outside the office.
Get to know each other outside the office, at the weekend. Your sexy, clean-shaven sales director, all smart and attentive in his suit and tie, might morph into a slob in a holey tracksuit at the weekend. What if his only other interest turns out to be souping up cars and he becomes incapable of communication whenever there's a match on TV? Your illusions could be cruelly shattered if you realize that the only thing you do have in common is work.
Keep work and home seperate
If you want any relationship to survive, you have to leave your little gripes and tiffs at home and don't bring them to the office (and vice versa - don't bring office gripes home!). Just as it would be a crying shame to spoil a night out by talking about work, it's not professional to hurl insults at each other in an open-plan office because he forgot to put the toilet seat down again.
Rivalry is another issue, should it ever arise: if a promotion, transfer or payrise doesn't go your way, it's not easy to be happy if someone so close to you gets the nod over you.