If and when I do get "down," the last thing on my mind is writing a song. Usually, being bummed just involves lying around on the couch and taking the bad weather personally. I only write songs when I feel good - or at least something approaching "good."
While the business of "collecting" lyrics and melodies happens all the time, when it's time to corral them all into something, I have to be in a good mood, ready and willing to work.
I write songs simply because I get a kick out of making them exist. I'm also sort of addicted to the recording studio, and making albums is my idea of fun.
On the whole, it bums me out that lyrics seem to be written as afterthoughts nowadays. Not sure why this is, but hopefully things will come around again and bands will once again want to "communicate" "ideas" with their audience, and not just content themselves with providing attitude and atmosphere with clichés on top.
Touring isn't traveling. Everyone should know that. And I would absolutely recommend a month of solo touring - that is, no driver, no merch person, no tour manager - to anyone in the position to do such a thing. But just once. You grow a lot in those situations, like when you spend a Christmas alone (which I also recommend). But, again, only once. That will be enough.