There are two main premises in the use of fonts, readability and legibility. To display an idea using few words, we operate under legibility. As seen here, various font choices can decrease the ease of reading the word.
When combining the two ideas of legibility and readability, you can turn a functional exercise of reading into a more artistic experience; however, this will decrease function. This directly operates under the Bauhaus premise that Form and Function are directly related, where affecting one will inadvertently affect the other.
Each letter has various design elements that one can manipulate which can cause varying visual experiences with the letters. In this case, the bold letters create a strong voice. Imagine if "I hate you" was thin. I wonder what they would be planning to do.
Fonts integrated into designs have this same effect and the designer should be aware of causing visual contrast between the elements of the image and the word itself. This font choice enhances the experience of the photograph.