It nearly killed him when...
Dick found out his identity.
Dick stepped up as Robin the first time.
When the Joker got to Dick before he could.
When Dick had his first nightmare.
Dick got sick.
Dick got hurt.
Parenthood was a jumble of emotions that Bruce felt ill-equipped for. Every day Dick tested his limits, pushed him to go the extra mile, and showed him just what it meant to truly love another person. His son was a gift from an angel- there was no other way to explain it. He was euphoric the majority of the time. That being said:
It nearly killed him when...
Dick smiled at him the first time.
Dick tried to save him for the first time- and succeeded
He watched Dick perfectly execute a move after months of practice
He watched Dick save the man who killed his parents instead of letting him drop to the ground
Dick got on the dean's list at Gotham Academy
Dick called him "Daddy" for the first time.