Because abel gave him the pendant when he was younger n the pendant meant alot to him (both ml n abel). The pendant was given to abel by his master(the guy who trained him n took him in) before his death
And I think that when Abel gave him the pendant they were on the Dragon's forest so there's no anyone could see that Abel gave the prince a pendant so I think he suspicion that new reincarnated abel.
That's why he got furious when he heard about it, plus reincarnated abel spouting too much moments that the prince and abel were only knew about it.
Can you remind me why he reacted like that about pendant?