Vancouver experiences a lot of rain for the same reason many similar locales throughout the world experience this phenomenon.
At the equator the wind tends to remain calm because of the currents of air flow throughout the world. As you move away from the equator the prevailing winds tend to come from the east, these are known as the trade winds. Further north, starting in the regions just outside the tropics, the prevailing winds come from the west. And even further away from the equator, near the poles, the prevailing winds again come from the east.
The reason for the directions of these winds has to do with the heating and cooling of the air at the surface of the earth, and coriolis forces due to the rotation of the Earth. These interactions cause the winds to be fairly steady throughout the Earth. In certain locations other effects can cause the winds at the surface to go in different directions than the prevailing winds but at higher altitudes these winds are fairly steady.
When air travels over the ocean it picks up large amounts of moisture from evaporation. As the air travels over land, and the land rises it, forces the air up, causing the air to cool. (This effect is called an “adiabatic cooling” if you wish to research further.) As the air cools the moisture in the air condenses falling as rain. This airflow and cooling is what causes the rainfall that is seen in many areas throughout the world. The air flowing from the east off of the southern Atlantic and rising up the Andes is what has created the rain forests of the Amazon basin. The air flowing from the west off of the Northern Atlantic, over the hills of Ireland, is what causes that island to be so green.
The mountains to the east of Vancouver create the same effect. As the winds off the Pacific blow ashore, the mountains force the air upward. The upslope wind cools and the moisture condenses, falling as rain.
Looking at the maps of the Earth we can see that similar conditions occur throughout the world. All coastal locations that experience an onshore wind have wet climates.
