Why might one get a headache almost every afternoon?

2 things come 2 mind. Stress from hours spent working, caring for others, watching the stock market, even being online for "fun" can lead to headaches. Also some people report feeling a headache when they have not eaten enough. Taking a break to relax and eat a healthful snack might help prevent a headache. Perhaps we all should try this :).
Many reasons. Many things can cause frequent headaches, and the description, location and associated symptoms help distinguish between some of them. Migraines are unusual every day.Tension headaches, and headaches associated with things like caffeine (especially withdrawal) can cause daily has as can regular use of pain meds (rebound headache). Refractive error of eyes and "sins/allergy" less common.
Other thoughts. I agree with dr. Yoder, and have a couple of other thoughts: eye strain headaches (are you working on a computer or squinting all morning?). Caffeine withdraw headaches (has your intake of coffee, tea, soda, chocolate changed?). Allergy/sinus (are you somewhere each afternoon where you're exposed to something (ie a cat, dust, etc) that you aren't around at other times? Hope that helps!