It is more clear how emotion works in the brain than intellect - the different 'main' emotions are obvious mentally and physically. However, how does a humans mind work intellectually? There must be certain types of attributes it associates with itself, and some of these are going to be more intellectually 'stimulating' to your mind than other attributes. This might also be true of other animals with less intellectual functioning than a human, however it is probably very different.
Just like how different types of sensory information can be stimulating to someone, different types of intellectual information are probably also stimulating to various degrees. School can be academically (or intellectually) rigorous, but what pieces of information from academic material are more rigorous?
The same information probably is just as rigorous outside of a school environment. Can I separate out information that is accompanied or 'forced' into someone by other emotional stress like if someone was under pressure at school to do well, or under pressure to figure out a solution otherwise?
Different types of information must be associated with different emotional architectures - i.e., when I think about this I feel this way. Some types of information are going to be more or less emotional or related to your personal identity. That is different from if the information uses more unconscious emotional processes. Information could use emotion to help to think about it (like my historical fact example), or the information could be emotional to the person or not as well.