Dirk is an oddly private man when it comes to expressing his feelings for someone. He has no trouble, say, walking down the street with his arm around his partner's waist, or holding hands, or various other forms of PDA, but when it comes to vocalizing it publicly [i.e. making random posts along the lines of 'I have the best boyfriend' or 'I love my boyfriend', etc.] he has difficulty. For whatever oddball reason he prefers to keep the verbal confessions and affirmations private, just as he dislikes discussing various aspects of his relationships with anyone not directly involved.
He's the man who will take his intended out for a romantic evening, have everything perfectly organized, and yet somehow manage find the most private yet lovely part of an otherwise public area to propose to them. It's likely that, if there are other people around, he wouldn't actually speak, he'd just go down on one knee and hold out the ring and trust that his partner understood; after all, how many other occasions involve kneeling and holding out a fancy-ass ring in a box.