~ことに (at the fact of, with verbs of emotion)

The pattern [clause]+ことに followed by a verb of emotion or reaction (驚く 'be surprised,' 喜ぶ 'rejoice,' 困惑する 'be confused,' 怒る 'be angered,' 文句を言う 'complain about,' 不満を言う 'voice dissatisfaction'), expressing 'at the fact that X.' The こと nominalizes the embedded action; に marks it as the trigger or target of the feeling or reaction. Covers both stative emotional reactions and verbal expressions of criticism.