What is the point of running into a 1v5?

My LOL experience tends to go in this direction a lot. Teammate runs into enemy team and dies in two seconds. They then get back at their team for not following up. Let me mention this is NOT Malphite or Jarvan or some tank jumping in. THAT expects follow up. This usually the mid laner, or a support, or someone who has no business leading the team fight. Not to mention they do it while their team is far away.

This always turns into angry, argumentative, flamers, and often quiters or feeders. So, I figured I should find someone who can offer a logical explanation. Why do people do that? Please, someone share their insight to this.