It all has to do with the temperature and the dew point.
Officially, the dew point is the temperature at which the air becomes 100% humidity, but it can informally be used as a comfort meter.
Around here, our summer dew points are generally in the 50s, while in the Midwest and back East, they can get into the 60s and even 70s.
But temperature also plays a big part of it. Your body doesn't really feel the humidity until the temperature gets over 70 degrees or so. So, for example, on a typical summer morning, the dew point in Seattle might be 55 degrees. The humidity will be high in the early morning because the morning low was likely close to 55 degrees -- but since it's that cool outside, you don't notice it as much.
But as the day warms up and we get into the 70s and low 80s, the humidity will drop because we'll get further away from the dew point, and it won't feel as humid. (There's a second factor here I'll bring up in a second.)
Now, to contrast, on the day we wrote this entry, the dew point was 76 in Orlando (which is incredibly oppressive) and 72 in Washington, D.C. So even though Orlando was in the 90s that day and D.C. was in the mid 80s, they're much closer to the dew point, so not only is it hot, but the humidity is higher, making it feel worse.
So, to recap, an 85 degree day with a 55 dew point feels a whole lot more comfortable than an 85 degree day with a 65 dew point.
Two other things:
1) The reason humidity makes it feel hotter is it makes it harder for your body to radiate heat. The body cools by sweating, which then evaporates into the air. The warmer molecules in those sweat droplets evaporate first, leaving the cooler ones behind and, in turn, make you feel cooler. If there is more moisture in the air, it's harder for water to evaporate, so it's harder to cool off.
2) We are incredibly lucky with our pattern here that it's either cool and humid or hot and dry. That's because when we get the humid breezes coming off the ocean, they're also much cooler, so it's cool and humid.
To get a hot day here, we need the winds comes from the east to push back the cool, ocean breezes. But those east winds come from the desert-like Eastern Washington. As an added bonus, that air sinks down the western slopes of the Cascade Mountains. As air sinks, it gets warmer and drier, so our hot days here are typically very dry -- just like the desert heat of southern California and Arizona.