system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what causes someone to develop a strong connection to someone, and what causes sadness/being hurt when rejected by the person.

I've always wanted to know if there's a scientific reason.
gpt
imagine you were building a house from bricks, whole bunch of bricks, some small, some large, altogether they make up the house...now imagine you remove one of the large bricks. if it's a foundational brick the whole house might fall apart and need to be rebuilt, if it's just part of the facade it will none the less leave a gaping hole in the building.

it's the same kind of thing with people. you incorporate them into your life, you build up your life in such a way that they are part of it, and when they leave that part is missing. 

rejection is kinda a different beast, if you put everything about you out there and someone rejects you it's a blow to the ego, they haven't just said no to your proposition, they have said no to you as a person, they do not see you as a valid mate. how could it not hurt. you have an image of yourself such that you believe this person should like you as much as you like them...and they don't... your image is wrong.