Why might someone have headaches every single day?

Many reasons: you may have a diet or environment change that leads to headaches, dehydration, elevated blood pressure, migraines (migranus continua or other), atypical face or head pains, sinusitis, brain tumor, scalp lesion, viral syndromes, the list goes on -- headaches are VERY common and many questions about onset, offset, timing, severity, location, family history, traumas, etc need to be asked to sort it