Bitterness is the complaint that sends most people looking for answers, and the good news is it almost always traces back to one of a small number of identifiable causes. A little bitterness is a natural part of coffee's flavour, caffeine is only a modest contributor to it, the bulk of perceived bitterness comes from compounds created during roasting and extraction rather than caffeine itself. Too much of it, though, usually means something specific went wrong.
The six usual causes
Over-extraction. Brewing too long, too fine a grind, or water too hot pulls out more than the pleasant compounds, including genuinely bitter ones. This is the single most common cause.
Grind size. Too fine for your method increases contact time and surface area, pushing extraction further than intended even if your brew time looks normal.
Water temperature. Water hotter than about 96°C extracts undesirable compounds more aggressively. If your kettle runs at a rolling boil, let it sit for 30 seconds before pouring.
Coffee-to-water ratio. Too much coffee for the water used concentrates bitterness along with everything else. If your brew tastes harsh rather than simply strong, check your ratio before assuming something else is wrong.
Dirty equipment. Old coffee oils build up in grinders, machines and brewing equipment and turn rancid over time, adding a bitter, stale edge that has nothing to do with your fresh grounds. Regular cleaning fixes this reliably.
Stale or poor-quality beans. Coffee past its best, or beans that were poorly roasted to begin with, often taste flatter and more bitter than fresh, well-roasted coffee.
Fixing it, in order
Start with the cheapest, fastest checks first: clean your equipment, check your water isn't too hot, and confirm your grind matches your method. Only after those are it worth adjusting ratio or reconsidering the roast itself.
The takeaway
Bitterness is rarely a mystery. Work through over-extraction, grind, temperature, ratio, equipment cleanliness and bean freshness in that order, and the cause is almost always one of the six, not something harder to pin down.
