When you're the immortal ruler of an entire kingdom, the most important quality to possess is self-control. No princess can rule her nation if her emotions rule her. Celestia knows this better than anypony, and mastered them long ago.
So long ago, she may have forgotten that a little bit of passion can come in handy now and again.
Celestia is in complete control of her emotions. That's not necessarily a good thing.
Luna always follows her heart. That's not necessarily a good thing.