While I'm certainly not doubting the value which fruits have as part of a well-balanced diet, they aren't always the most diet friendly choice. This is because many fruits have loads of sugar in them.
When we make the choice to have fruits as a large part of our diet, we have to analyze how much sugar they contain and properly adjust portion sizes. For example, mango is an incredibly popular (and delicious) fruit, but did you know that just one cup of chopped mangoes contains 23g of sugar? That's just three grams of sugar off from a can of Coca-Cola!
However, one cup of whole strawberries has just 7g of sugar and is more nutritionally dense.
Don't be fooled by those who say "but it's sugar from fruit, the healthy kind!" Sugar is calories, no matter what form it comes in, and if you're tight on your caloric budget, then you have to make sure you're not overdoing it with the fruit.