I left most things blank, since they were either summer research, volunteer stuff or "leader of club xyz in med school" and everyone knows how that stuff goes. Nobody is going to ask you in an interview "so why did you stop volunteering at the homeless shelter?"... "uh, because I started this other volunteer thingy"... The only thing I filled in was saying how I left my pre-med-school job position to I could go to med school. If something negative happened that you think looks odd, I would use that spot to explain (ie: left because of death in family).