It's almost crazy to me that you wouldn't call strawberries sour. Strawberries taste quite sour to me, and quite sweet as well. I've always thought of sourness as relating to acidity (strawberries and grapefruits actually have pretty similar pH's). I perceive bitterness to be entirely different (strawberries are not bitter, grapefruits are slightly to moderately bitter, depending on the grapefruit, kale is very bitter to me but not at all sour).