This retrospective study aimed to investigate the influence of patient behavior toward dental treatment on the presence of adjacent (sound, decayed, or filled) proximal surfaces in the primary molars. The study included a records review of 2226 pediatric patients. Records were eligible if the patients were in primary or mixed dentition, and if they included a complete set of bitewings and periapical intraoral radiographs of all their primary molars. Four calibrated reviewers used intraoral radiographs to record the sound, decayed, and filled proximal distal surfaces of the first primary molars and the mesial surfaces of the second primary molars. Demographical variables were reported from patients’ medical records including gender, age, medical history, nationality, and patient behavior. Patients’ behaviors were evaluated using Frankl’s Behavior Rating Scale. The Pearson correlation was used to test the association between the sound, decayed, and filled distal surfaces of the first primary molars and the mesial surface of the second primary molar. High correlations of ≥0.6 were further tested in simple and multiple linear regression models to test the influence of patients’ behaviors on these correlations after adjusting for demographical factors. All tests were performed at 5% significance level A. A total of 1194 records met the inclusion criteria and were analyzed. There was a high positive correlation between the sound, decayed, and filled distal surfaces of the first primary molar and similar surfaces on the mesial of the second primary molars (0.66, 0.61, and 0.60, respectively). Compared to cooperative patients, the adjusted estimate of the mean decayed mesial surface of the second primary molar increased significantly for non-cooperative patients by 0.1 (95% CI = 0.16–0.53). On the contrary, the adjusted estimate of the mean sound mesial surfaces of the second primary molars decreased significantly by 0.09 for non-cooperative patients, compared to those who were cooperative (95% CI = −0.52–−0.15). Patient cooperation did not significantly influence the mean of the filled mesial surfaces of the second primary molars (adjusted B = 0.01, 95% CI = −0.05–0.09). Educating parents and clinicians about the influence of children’s behaviors on oral health is highly encouraged to improve treatment outcomes and reduce the progression of dental caries. The implementation of specific behavior management techniques is also important to reduce dental fear and anxiety.