Being sincere to others doesn't mean saying everything, but rather showing a friendly attitude and observing their reactions. There are only three types of people: the first type responds with a friendly attitude as well and does not change over time. The second type thinks you're easy to bully and becomes arrogant. The third type is friendly at first but gradually starts to want to manipulate. Watch their reactions and then adjust your strategy. You can gradually deepen your interactions with the first type, become more distant and refuse their requests when necessary with the second type, and maintain distance and assert your strength at the right time with the third type.