Based on the question linked above, assume dragons are scientifically plausible. You may change what you need about the accepted answer to the above question if it helps you answer this one, but stay within the realm of science.
Many myths portray dragons as more than just ferocious beasts - as tricksters, guards, or big baddies out to get the hero. This implies intelligence.
The problem with making humongous, firebreathing beasts intelligent is that there is seemingly no need - they are already fit to survive and reproduce (related fitness discussion is in the answers of What would cause turkeys to be intelligent?). It's difficult to come up with an evolutionary pressure that would select for intelligence when most dragons will survive regardless.
What pressures or events would cause dragons to evolve human-level intelligence?
Note: I do not require communication, tool usage, or other human-like abilities; simple self-awareness is acceptable although other processes may be necessary first.
If it helps, these are my thoughts so far:
Humans are considered to have developed intelligence as a result of
- Language use and collective learning
- Tool usage
- The use of fire, which allows the stomach to work less, and provides the energy for the brain to work more
While I don't require the first two conditions in answers, dragons have achieved condition #3. Perhaps this is a starting point for your answers.