I think a better way of putting it is: "don't get too emotionally invested". Writing code I often times get emotionally invested in it's success, but with a job you have to create that clear line of "I care, but I'm not emotionally invested". I really want to fix that bug in the system but if 5PM hits, I don't have the emotional investment to keep going into the night like with a side project, I can get to it in the morning.