The single-use stairs

Screaming, followed by crying from my son (2.5 years old)
I rush out, thinking he's been hurt on the stairs...
 
Me: What's wrong?
C (2.5 years old): Bran (our dog) went downstairs.
Me (slightly confused): But Bran is allowed to go downstairs.
C (through sobs): But *I* wanted to go downstairs.