There are actually cases of people getting SAD from the over abundance of sunlight in the summer (http://cliffmass.blogspot.com/2012/10/depression-from-too-mu...)
http://www.seattleweatherblog.com/rain/record-dryness-windin...
I used to live a few hundred kilometers away from Seattle in the same temperature area, I can say for sure I'm looking forward to this summer!
The relentless gray of which you speak is true of months like December and January.
I'll also take nonstop rain from Nov-Feb any day over the snowy winters in the Midwestern states where I grew up.
Finally, the key to not letting your plans get ruined by rain is to always plan for the possibility of rain. =)
When I travel, I always emphasize the rain, the cloud cover, the risk of death by volcano, the very real likelihood very overdue The Big One earthquake, how traffic sucks, etc.
http://en.wikipedia.org/wiki/Emmett_Watson http://en.wikipedia.org/wiki/Lesser_Seattle
That said, sometimes it does rain a lot in Seattle. Some years ago there was a stretch of 35 consecutive days of rain.