There are several reasons why reading mathematics is hard. In part it's because the language of mathematics is extremely dense, and you must know the exact meaning of the words and symbols you encounter. Even a mathematician, reading in an unfamiliar topic area, may be forced to stop frequently to look up words and check meanings.

But perhaps the biggest problems is that you must bring the right attitude and approach to the act of reading mathematics. You will have to stop and look up unfamiliar terms. And in many cases you'll have to transfer what you are reading to paper --and work through it-- to make sure that you are getting the point.And you must organize your thinking. A lot of math requires the construction of a mental structure around the topic. This is typically not the kind of reading you can do as you drift off to sleep at night.

A good essay on how to dissect a math paper: How to Read a Mathematics Paper

On the other side of the coin you may find it interesting and instructive to consider the process of writing about mathematics. Learning how to write mathematics will help you understand what you read.

A Guide to Writing in Mathematics Classes