Seattle gets a high quantity of rain due to its location between the Pacific Ocean and the Cascade Mountain range. The winds carry moisture from ocean towards the land. The Cascade mountain range contains tall mountains, over 10,000 ft in height. For ex., Mt. Adams is 12,307 ft above the ground and Mt. Rainier is 14,410 ft above sea level. Moisture-carrying winds can not transcend these mountains or go through them. So he phenomena of "Rain Shadow" takes place. As the winds rise up, adiabatic cooling takes place, forcing them to lose moisture on the windward side of the mountain and this area gets a lot of rain. On the other hand, the wind on the other side of mountain (leeward side) have very less moisture and so this region would get very little precipitation.
Seattle lies on the windward side of the Cascades and hence gets so much rain. The same is experienced by Portland, Oregon.
Seattle is very rainy because of the location near the ocean! The sun evaporates the water, which accumulates in the air, and when the clouds have too much water, it starts to rain. The ocean is such a large body of water that a lot of rain is to be expected. There are other factors as well however, but you cannot have rain without water first!