People in our lives: Reasons and timing

They say that people do not come into our lives by chance. One person brings joy, another brings pain, a third gives us life experience, and a fourth teaches us something new. However, it is not always clear how long a person will stay in your life and why they came into it in the first place. Or did you let them in yourself? And sometimes it happens that it seems like they have already done something for you, given you something, and you just don't want them to leave. What to do in such cases?