Phantom always hung out with Fenton's friends. Fenton always disappeared before Phantom appeared. Fenton, Phantom - so obvious. If Dash didn't stop them, who knew how many would blab the ghost's secret?
Dash, King of Casper High, kept them quiet. It was to help Phantom - maintain the masquerade. It was to protect Paulina, Queen of his kingdom - she'd just be hurt if she found out her beloved ghost-boy was a dweeb.
It was not because Phantom had stolen his Queen's heart. Not revenge.
Dash walked up to Fenton, smacking his fists together. Just maintaining the masquerade.