Smaug is a dragon and dragons are attracted to gold & stuff. Thror was told to keep his treasure hidden in case the dragon saw him. But Thror refused and so the dragon stole his gold and killed the dwarves.
Smaug, one of the last dragons if not the last, loves gold, as stated above, Thror decided to not heed the warnings given to him AND i’m sure the dragon knew there was gold i mean, one of the most successful... races well say, would definitely have lots of gold either from tribute, war, plundering, and of course gifts to remain allies! Hope this helps
