Oliver Queen never cries.
So the times that Diggle catches him doing it, it breaks his heart even more because he keeps his emotions to himself, so when he cries, he really cries.
The fact that Oliver tries to hide it from Felicity and Diggle, makes Diggle want to slap the man and say, "Oliver, it's ok to cry, and if you keep hiding it, I will personally put the fear of god in you."
But Diggle stays silent.
He watches Oliver as he stares at the floor, his head bent down slightly you you can't see the hot tears running down his face,
Depriving himself of air as he tries to control his labored breathing,
Trying to make sure we don't hear him sob.
Diggle just watches,
Ouside, Diggle's face is calm,stern,
Inside, he's crying too.
Three times he's caught Ollie crying.
His teammate,
His friend,
His brother.
Diggle is NOT going to stand by and watch it happen again.