What meas for " I wouldn’t be in this job unless I were an optimist!"

I meet a phrase: 'But I’m fairly optimistic. I wouldn’t be in this job unless I were an optimist!'. Here I don't the what the means I wouldn't be in this job unless..... from my understanding on my poor english. I think he say that 'unless I am optimist, I don't like to do this job' . this is right?

Thanks for any comments