Grapefruit, in season September through April, can be extremely bitter, even when it's at its juiciest. Some people douse grapefruit in sugar or an artificial sweetener to cut through the tart acidity; however, a sprinkle of salt, although it seems strange, is an effective way to neutralize its bitterness.
Try it out at home by cutting the grapefruit into bite-size wedges, and then sprinkle it with salt. Keep your salted hand at about a two-foot distance, so the salt disperses evenly. As you eat the grapefruit, you'll be amazed by how much sweeter it tastes.