It's even worse when he reveals his Tragic Keepsake—her necklace. Which means Spooner either saw her body in the morgue, or Sarah's remaining family members gave it to him at the funeral. Poor guy.
"She wanted to be a dentist. (chuckles sadly) What the hell kinda twelve-year-old wants to be a dentist?"
Even more crushing is the almost clinical way that Spooner recounts what happened to Sarah. Listen to his voice. He starts out so calm and factual that one can easily assume he's told this story before, most likely to Sarah's family and to the police after he recovered. Furthermore, he knew the exact percent that the robot calculated when it chose to save him over Sarah, meaning that Spooner probably demanded that the robot tell him why it didn't listen to him, which could only have broken him further. The way he gets choked up by the end of the story is really what seals it because he's clearly been repressing his feelings so hard that it causes the recurring nightmares and Susan's question brought it all rushing up to the surface.
Spooner: That was someone's baby. 11% is more than enough.
More tragic on that note—Spooner is someone's baby (his grandmother's) too, but the way he talks about it means he's clearly stopped considering himself as worth anything anymore.
Spooner used to be married. Odds are that he still was during the time of Sarah's accident, and then that trauma was so great that he couldn't hold his marriage with Marcy together anymore.
The truck driver who was responsible for the accident didn't do it out of any malicious intent or substance abuse; he was just an average guy with a wife and kids working a double-shift and fell asleep at the wheel due to exhaustion. Spooner even says that he wasn't a bad guy, just someone doing his job who got unlucky. It's unknown if he survived the accident as well, but either way it's a sad fate. If he didn't, then he is one of the unfortunate victims and leaves behind a widow and children; if he did, then he is likely as haunted by that night even more so than Spooner.
Adding to that, when Spooner first stumbles upon the NS-4's getting destroyed by their newer models, one grabs onto his ankle, short-circuiting and missing its legs to say one word before shutting down. "Run...!"
Sonny finally admitting he was instructed by Lanning to kill him. He looks like he's on the verge of tears he cannot shed as he struggles to comprehend his own feelings about being asked to kill his beloved father figure. Spooner's sympathy makes it more poignant.
Sonny: He said I had to promise. Promise to do one favor for him. He made me swear, before he'd tell me what it is he wanted me to do. He made me swear.