It is a mindset thing. There are countries where it rains all the time, if you didn't go out and do stuff in the rain, you would never leave the house. There are slab climbs in britain that go up subtle grooves in the surface of the cliff that are rivers when it rains, people boast about who climbed it in the deepest torrent.

That said, rain, cold, headwinds, and rough camping can get me down... And, you have to seriously consider your route for safety. If you are taking a route with lots of cars passing, and where the rain will affect your safety big time, then you need to consider alternative routes.