Something I hear a lot is "their smile is contagious." This isn't meant in a literal way, but it means that being around someone who's smiling makes you want to smile too. Humans are very social creatures, and so we reflect each other's emotions in order to build relationships and bonds with each other. Sometimes this can negatively affect relationships. For example, if one friend is always sad, the other friend might find it hard to be happy around that friend. Since being sad all the time is a negative feeling, this negative feeling will be associated with the friend that's always sad, which will damage their relationship.
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