This narration has been narrated both in Shia and Sunni books and the answer has been mentioned in the narration itself.
" Imam Hussein (AS) and Imam Hasan (AS) were wrestling together before the messenger of Allah, and Prophet was cheering and heartening Imam Hasan (AS) and was telling him to take hold of Imam Hussein (AS).
Hazrat Fatimah (AS) who was also present there said: why do you cheer Hasan who is older than Hussein?
The messenger of Allah said: I am cheering Hasan, because the "Gabriel" is cheering Hussein, and he is telling Hussein to take hold of Hasan.
source : erfan.ir