Most of my songs emerge when I'm doing something repetitive & rhythmic where I can just let my mind wander around & kick up odd phrases & stories. So a lot of songs gestate as I'm cycling to work, then I frantically type them up when I get there.

The other good time is when just playing randomly on guitar - sometimes the words just roll off the tongue.

So I'd definitely agree that they tend to "just happen".
I have tried following a set formal method, just as an exercise, and was amazed to find it worked fairly well - but being fundamentally lazy I prefer to lie in wait.

I have also tried collaborating with a friend - but it didn't suit. I was too diffident to make suggestions or constructively criticise - and such ideas that I did selfishly kept to myself. :)