Trust
Ace always found it difficult to trust other people.
He was not like his two younger brothers.
While Sabo always remained suspicious of strangers – always guarding himself – he somehow simultaneously managed to fool them into thinking he trusted them.
And Luffy – Luffy trusted everybody instantly. It caused Ace more headaches than he was willing to admit.
But Ace never trusted other people – aside from his younger brothers. His brothers were special, and they would never betray him.
Even trusting his handpicked crewmembers was difficult sometimes.
So, trusting the Whitebeards, after finally joining them, proved to be a real challenge.