Listen.... the 'Ace's little brother' and not my or our little brother part ... Luffy is his baby brother too but imagine how hurt Luffy probably is when he realises that ever since he met Sabo again Sabo has only been refer to him as ace's little brother and not 'my little brother' BC THAT IS WHAT YOU MADE ME THINK I AM IN PAIN
“Luffy is Ace’s last will, I won’t let you hurt him! Ace gave his life for Luffy, I won’t let that sacrifice be in vain!”It all sounds nice and stuff, but Luffy would soon realise that Sabo sees him as nothing more than an extension of Ace, which would suck so much. He’d probably figure thats why Sabo refuses to hug him, why Sabo never woke him up to say goodbye. He’s nothing more than a duty for Sabo, an obligation that he needs to fulfill. One was before eating the flame fruit one was after, but all it means that Sabo manages to be even more obsessed while simultaneously being emotionally disconnected from Luffy. (If you wanted to take this theory all the way, you could even make the argument that Sabo has become so obsessed with seeing the world through Ace’s eyes that he slowly starts to lose his own sense of identity)