Presumably there are multiple additional safeguards in place (TCAS etc) to ensure that it is not down to the error of one single ATCO, should the worst happen?
Human factors is something that NATS covers in detail at the college (where I currently am), and the pattern that starts to emerge is that it is rarely one single thing that causes a mid-air collision. There is normally a string of errors that worsen the situation (see
here for a recent example). Safeguards are more in the form of aids for the situational awareness of the controller, and TCAS as you mentioned.
As for the original question, I want to do the job because it involves working shifts, you can't take it home with you, it will make me use my brain and the I suppose I like the responsibility.
Ginger