This study aimed to characterize mental health discourse manifested in user comments on YouTube videos concerning depression and bipolar disorder. Specifically, the study analyzed how discussions related to personal experience, stigma, and community building are expressed within digital interactions. A total of 100 videos were identified using the keywords “depression” and “bipolar disorder”. Using YouTube API, basic information of the videos(channel, viewcounts, title and etc) and 16,241 comments were collected. Most of the collected videos were produced by individual doctors, media outlets and general hospital, which focused on providing information on the treatment, diagnosis and symptoms. After removing non-informative, duplicated, or commercial posts, 13,183 comments were analyzed. Using GPT-5, comments were classified into eight predefined categories. “Personal experience” was the most frequent category (9,551), followed by “Needs for Community Building : Emotional” (3,844) and “Needs for Community Building : Instrumental” (2,875). In contrast, actual “Community Building : Emotional Support” (1,347) and “Community Building : Instrumental Support” (551) appeared less frequently. Stigma-related discourse occurred in “Social Stigma” (1,719) and “Internalized Stigma” (1,186), while “Non-evidence-based Advice” appeared in 1,148 comments. Co-occurrence analysis showed strong associations between “Personal Experience” and “Needs for Community Building : Emotional” (.56), “Needs for Community Building : Instrumental” (.36), and “Internalized Stigma” (.33). The comment space of expert mental health videos operated primarily as an experience- and emotion-centered venue where users expressed substantial emotional and informational needs but received limited reciprocal support. These findings underscore the necessity of digital mental health environments that integrate evidence-based information with structured emotional and instrumental support frameworks.