Since I was a kid, I've thought I was "prone to migraines", and ascribed various triggers to them - sun exposure, heat, physical exertion, mental exertion, etc. I'd get a migraine sometimes after a long hike on a weekend - and also a long business meeting entirely indoors in an air-conditioned space.

Only when I was around 35, did I figure something out. All these situations lead to me getting dehydrated without any obvious accompanying feeling of thirst. Hiking all day will do it - walking around an outdoor shopping mall on a hot afternoon - or sitting in a business meeting focused on the work at hand and forgetting to drink. And all these situations lead to a migraine - my only "migraine" trigger is simple dehydration, nothing more complicated.

The weird thing is, it took me a long time (decades) to put this together, because I just figured that I couldn't be dehydrated if I wasn't thirsty, and I had no association between "feeling thirsty" and getting a migraine.

I get what I consider normally thirsty in other circumstances, but somehow there's a failure mode where my body doesn't warn me. So now I just remember to chug lots of water (and electrolytes) if I'm exerting myself even if I don't really feel thirsty, and I can systematically avoid triggering migraines.

Now that I understand it the association is quite clear and obvious in retrospect.