All true dragons polymorph into humans (with exception of white dragons, who don't really like to do that).
Very few though actually would be interested in humans (and other minor mortal races).
Even metallic dragons known to be somewhat arrogant, and hardly any dragon would see any non-dragon as equal.
Are you sure, just checked the books and Silver Dragons have a Change Shape section while Red Dragons do not:
Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).
In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.
and later it talks about how they like being humaniod:
Friends of the Small Races. Silver dragons enjoy the company of other silver dragons. Their only true friendships outside their own kin arise in the company of humanoids, and many silver dragons spend as much time in humanoid form as they do in draconic form. A silver dragon adopts a benign humanoid persona such as a kindly old sage or a young wanderer, and it often has mortal companions with whom it develops strong friendships.
Silver dragons must step away from their humanoid lives on a regular basis, returning to their true forms to mate and rear offspring, or to tend to their hoards and personal affairs. Because many lose track of time while away, they sometimes return to find that their companions have grown old or died. Silver dragons often end up befriending several generations of humanoids within a single family as a result.
