Part of it is built into the human psyche. It's in our genes we are social animals.
While I do not know this for a fact but studies have shown that solitary confinement over years and extended time periods have driven people insane.
But that is solitary confinement truly being alone would be different altogether. And I feel that the definition of insane we have to be more clearly defined. Shirley in your own mind you talk to yourself can you imagine what that would be like if you were truly alone for say 10 years no contact with any other human would you be able to communicate with a say the birds or any other type of creature that you happen to be around you would surely have the time devoted to learning their language.
Sadness is usually an emotion that is selfish meaning you cry and you are sad for yourself it's because you miss and you are without you generally are not sad for the other person and as far as I know the only way to turn that off is to not care.
personally myself I will never do that therefore I will always get hurt and be sad but I find that I'm a better person caring about other people that I am not caring I choose to love and not to be indifferent