The saying, "out of sight, out of mind," can easily summarize the social psychological phenomenon known as the rule of interpersonal attraction.

The correct answer should be proximity.
Proximity refers to physical closeness of people. The saying 'out of sight, out of mind' means that we are likely to forget about people who are far away from us than if they were near us all the time. Even if we used to be close, when they are not with us, we tend not to feel the same closeness. 

Hamlet's fourth soliloquy is probably his most famous one - "to be or not to be."it is rather different from his previous soliloquies because now he is contemplating something that didn't come up before - suicide. he is thinking whether it is worth it to fight to find his father's murderer when in the end all of them are going to die, so why not die sooner than later. he is contemplating if he should go on fighting or just kill himself and end it all, which is why this soliloquy in particular is rather depressing, hesitant, and gives a deeper insight into hamlet's madness.