He doesn’t mean to cry, but the more he holds back, the more painful it is, so he cries. He’s tried to be brave for so long that he’s forgotten what it’s like to be weak.
He’s wailing now, enough to make him hiccup in between breaths, because all those years spent being angry and frustrated over being betrayed by his own brothers, betrayed by his master and seemingly forgotten by everyone, all that time has become useless. It’s been rendered useless because in the end, he had mattered to his family, because their guilt shows on their faces now as they try to avoid his eyes. He matters to his master now, because he’s become the king’s right-hand man. And even though it took some time to realise, he has always been remembered by the tears that soak his father’s face and the words that leave the cupbearer’s mouth.
It makes all that anger and frustration he’d kept inside useless, so he purges it from his body, and now cries out of hope for a full reconciliation with those he’s never forgotten.