OK, total speculation coming here--but if I were going to explain this in a story, I'd say this: perhaps Changelings have a certain "default" humanoid form that is part of a sort of racial memory. (A form which, interestingly, resembles Salome Jens' OTHER Trek role...) Unskilled Changelings may automatically wind up in a form of this default mode when they first try to approximate humanoids. Odo and Laas would make sense under this explanation. As for the others...maybe that's how they think of themselves when choosing humanoid form. Maybe it's not humanity it occurs to them to approximate, but that first race that seeded the galaxy with all of the other humanoids. (The one that left the message in "The Chase".)
Total personal theory, but I think it would at least work as a book/fanfic explanation.