Why do we think of a person?

 1- Because the other person is thinking of you 

Thinking about a person constantly and not being able to forget him is because this person is constantly thinking of you and that you occupy a large part of his thinking, and this is called telepathy or addressing spirits, despite the lack of this reason, but it is largely proven in psychology.

2- An old relationship:

We cannot think of someone with whom we do not have a strong relationship, whether old or new.

3- The distance between the two of you: 

When you get used to the presence of a certain person in your life and he is always close to you, you will find yourself thinking about him subconsciously, because the mind and the eye get used to his presence, and he will be present in your imagination despite his distance from you, and this means that you began to relate to this person.