Firstly, it could be that this person has always been a good listener but was not comfortable opening up or initiating conversations until they felt a strong connection with you. As your friendship deepens, they may feel more comfortable sharing their thoughts and feelings with you, leading to a stronger bond.
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Secondly, it is possible that this person has undergone changes in their life that have made them more open and communicative. For example, they may have started therapy, undergone personal growth, or experienced a life event that has prompted them to reevaluate their relationships and communication style.
Finally, it could be that you have a natural rapport with this person that allows for easy conversation and a deep connection. Sometimes, two people just "click" and feel comfortable sharing with one another even if one of them is not typically talkative.
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Regardless of the reason, it is a positive sign that this person has become your best friend. It shows that they trust you and value your friendship enough to open up to you, and that you have a strong connection that goes beyond surface-level interactions.