Fandoms are good because they can help boost a person's self-esteem
When a person is part of a fandom, this can inspire a greater sense of self-esteem. Since they are reinforced by the people around them who also share the same adoration for a given subject, the reinforcement they receive positively affects their mental health.
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Fandom culture is enriching to its participants because it affords them the ability to build upon and even improve their self-esteem, as well as contribute to sound mental health. Fans can experience greater levels of happiness as a result of their devotion to a figure or franchise. The happiness they feel serves to act as a barrier against depression and estrangement. For example, take being a fan of a sports team. If the fan is far from home, identifying with a sports team can help them feel less estranged, bringing about a positive sense of self-esteem and well-being.  The relationships fandoms create are fundamental since participants are not as lonely, helping them feel more satisfied with their lives compared to those who are not into things like sports. Fans are also able to enjoy more access to beneficial resources such as social support.  Access to resources like social support is facilitated by the other fans who also participate in the fandom culture. Since they all share the same passion for an entity or organization, this creates a built-in connection to them that allows fans to access beneficial resources.  All in all, a fan's devotion to a subject of their choosing can lead enhance their self-esteem and also contribute towards better mental health as a result of being part of a collective group that shares their passions.
On the other hand, fandom culture might be more detrimental to a person's mental health than previously thought. Participating in fandom can lead to obsessive behaviors, even resulting in taking part in actions that threaten the safety of others (for example, when a Harry Potter enthusiast stabbed someone for saving a seat at a Marvel panel).  As one might see, having deep devotions to a figure or franchise can elicit violent behavior over seemingly trivial occurrences. Though it predominantly has to do with a fan's level of investment or interest in the subject, these are the kinds of behaviors fandom culture can foster.
Rejecting the premises