Most mathematics textbooks are very dry. Basically every page is full of text with no pictures, and every heading is either "Definition", "Theorem", "Proof", etc.
Your brain craves novelty. It’s always searching, scanning, waiting for something unusual. It doesn’t bother saving the boring things; they never make it past the “this is obviously not important” filter.
Research indicates that: Images are far more memorable than words alone, and make learning much more effective (up to 89% improvement in recall and transfer studies). It also makes things more understandable. Putting the words within or near the graphics they relate to, rather than on the bottom or on another page, and learners will be up to twice as likely to solve problems related to the content. In recent studies, students performed up to 40% better on post-learning tests if the content spoke directly to the reader, using a first-person, conversational style rather than taking a formal tone. A conversational style helps because people tend to pay more attention when they perceive that they’re in a conversation, since they’re expected to follow along and hold up their end. On the other hand, if the writing style is formal and dry, your brain perceives it as boring.