Eri got angry at Tenma because of jealousy.
Tenma didn't know why but tried to apologize anyway.
When Eri and Harima where on a "date" in town and got stuck there due to lack of money, Tenma waited outside Eri's house in the snow for 24 hours.
Eri wanted to apologize to Tenma for leaving her out in the snow, but for some reason, unkown to even her, said it was her own fault.
Cue Yakumo Bitch Slap.
Pain and Regret.