Testing Christmas Lights

Why is it you put up a tree and some of the lights just won't work. That is Tim's job, checking the lights. Usually he is in a good mood, but if he can't get the lights working after 30 minutes or so the "Grinch" appears. LOL. Thank goodness he never stays around too long. Finally get all the lights working and fluff the tree. These are our 2 living room trees. Ready to decorate.