@Background Pony #186F  
When an episode calls for it, Celestia shows compassion for Twilight. Twilight is the central character and Celestia’s personal student, so of course that’s how it works. The show wouldn’t have the same feel without it.
 
Thus far, we haven’t really had a Celestia-centered episode, so complaining about her bond with Luna seems out of place. I mean, we’ve had several Luna-centered episodes already and none of them touched on her relationship with her sister, did they?
 
My take on things is that if, in a Luna-centered episode, if Luna herself doesn’t mope or groan about her relationship with her elder sister, it’s likely not a problem and they’ve just as likely reconciled.
 
Moaning about either of the Royal Sisters (Celestia or Luna) harboring some sort of animosity towards each other just seems childish to me.
 
They’re sisters.
 
They’re both more than 1,000 years old.
 
If they wanted to drag on and on about how upset they are with each other, we would have seen it already. And let us not forget them squabbling during Cranky’s wedding, which they quickly got over once the ceremony actually commenced, holding each other’s hooves and everything.
 
They love each other, just as sisters should. And knowing how compassionate Celestia is towards Twilight, her student, it isn’t hard to imagine that she’s just as compassionate towards her blood sister.

Beside the second episode, Celestia never actually showed that she loves Luna. She rather hugs Twilight, who surely needs it less, as she already has her friends, brother, Cadence, parents and Spike who would stand there for her. So this song seems out of character for me.

If this song didn’t make you tear up even a little, then you have no soul.

@Barhandar  
Also possible.  
Maybe, at that time and place, Luna could not be redeemed. Maybe her exile on the moon was somehow necessary to allow for her redemption.
 
Or maybe Celestia just reacted to a threat to all Equestria as quickly as possible with the one weapon she had, and only afterward realized what she had done.

@monolyth  
@Timberwolf  
The wielder just triggers the Elements and doesn’t control what they will actually DO.

@monolyth  
She couldn’t bring out theri true power, let alone enough to save luna. Not alone.

@monolyth  

If the helements of armory could revert nightmare moon back into luna, why didn’t she just do that the first time?