Christmas lights baffle me!

Why is it when you take out the Christmas lights from the year before they no longer work?
All the lights were working when I took them down last year, but on Sunday when I began working on my Christmas light display I discovered some of them were no longer functioning.
At first I thought I was getting lucky. I plugged in several strands and they were all lighting up. Then I hit the big wreathes I put up every year.
They had red and green lights on them. Neither wreath's green lights worked. So I decided to go with both of them with just red lights which worked before I put them up.
I plugged in some lit garland and it all lit up! Woo hoo! I was set.
Out I went. I had the garland strung about 20 feet when I noticed that the next 20 feet were no longer all lit up. So I began taking the lights off the garland, grumbling all the while. Then put new lights on them. It was tedious and my fingers were cold.
Later on I went up on the roof to put up the wreathes on the front of the house. When I plugged them in... only one worked at all.... grrrrrr! I was not happy. So I decided that I would try wiggling wires and within a while I found the short. They were now both working! Yaaay!
I threw out several sets of lights. I just don't get why they work for a month in December and then when you go to put them back up they are riddled with bulbs that no longer twinkle. For the amount they cost you would think they would last forever.
–Until then, Ken