Because dragons tend to ignite ordinary bedding, they must find a soft metal to sleep on, and gold proves most comfortable for them.
9 seconds sound clip from the The Flight of Dragons movie soundboard.
You can hear this line at 00:30:49.256 in the Blu-ray version of the movie.
Quote context
[...]
- My comrades... My Cronies... The entire... flight they all... All turned against me.
- Because I wouldn't denounce you and join Ommadon.
- The other dragons have gone over to Ommadon?
- Aye. Aye he's cast a spell on them all.
- A flight of dragons to protect his crown.
- What a strange use for gold!
- Ah... hence the dragons need for a hoard.
- Ahem. Perhaps we' better continue this conversation in my study, less chance of interruption there.
- The Last Realm Of Magic, My own personal dream.
[...]