Why is the human reaction to headaches and migraines to increase irritability and decrease patience?

Stress

Explanation:

The human body can only handle so much stress at a time. The more stress you're already feeling, the closer you are to going over your threshold, and therefore the smaller the minimum stressor needed to send you over.

Any kind of pain is a stressor, but headaches seem particularly stressful, my guess is because the pain is centered directly in your brain, but that's not scientific at all.

tl;dr: The more water you have in your cup to start with, the less you need tl add to make it spill over.

yes. i think it is for sure.