Ego. Ego makes someone think that they are the best at absolutely everything, giving complete disregard to humanity. It completely disconnects a person from the world.
Second would be depression. Depression makes the brightest days seem hopeless and usually causes a person to spin into a downward spiral where they get stuck. Getting stuck is one of the worst things in the world, as a person feels as if they are not capable of doing anything. Then the loneliness showers in as we try to grasp for air, yet are suffocated by our own misconceived notions of self-worthlessness.
Originally posted on Quora.
Leonard Kim consults startups and writes books like The Etiquette of Social Media: How to Connect and Respond to Others in the World of Social Media