Christmas Lights Sets Keep Burning Out?

Most of the time it's from age. No matter how good you take care of the whole set and replace the bulbs as they burn out here and there, after a few years the little wires on the bulb start to oxidize and the oxidization eventually works its way up the small wires into the hermetically sealed envelope of the bulbs which causes the vacuum to leak out and makes the filament blow out. This happens to every bulb almost at the same time. You take them all down the previous year when they were all working and they get stored in the attic, basement, garage etc.

which is when they get exposed to the moisture and humidity during the spring and summer months and when you put them up the following year you're thinking "They were all working when I took them down last year! What happened?!" Unfortunately the only remedy is to buy all new bulbs or replace the entire string.