I agree with the others....for one reason or another that part of the wall/ceiling is staying cooler than it might. It drops below the dew point and voila, water.
A perfect roof needs no ice and water shield, but a perfect roof is a tricky thing. The idea of proper insulation and venting is two fold: Keep heat in the house, keep the roof cold. Any heat that escapes from the house should be vented away before it warms the roof. That's when the problems start, the roof get's heated, snow melts & runs down until it hits the cold roof near the edge, then it freezes. The ice will get thicker and thicker, eventually making a dam. Once the dam gets high enough the water leaks though the shingles. This is where ice & water shield does it's job. So, good insulation, soffits vented, roof ridge vented is the answer. They make stuff called "proper vent" that goes between the roof rafters. It's stiff styrofoam zig-zag stuff & it's job is to keep insulation from touching the roof and to maintain a 1 1/2" vent space so air can move from the soffit to the attic area. If you have a cathedral ceiling, the proper vent should run the entire distance from soffit to roof vent.