Strawberries taste differently depending on what stage of their development they are.
When they are the most ripe, they tend to taste quite sweet, although they can taste sour to some. It is important to keep in mind that when you are picking them if they taste sour, they are not yet ripe.
Typically you will find that this fruit is a mix of both sweet and sour tastes, though it does depend on the person. What one person finds sour another may conclude to be sweet. Generally speaking though, strawberries are â€œsupposedâ€ to taste sweet if they are fully ripened.