Answer:
1. Shakespeare uses a huge vocabulary, far larger than anyone else including the audiences who saw his plays for the first time in the 16th and 17th centuries. There are inevitably going to be lots of words the reader does not know.
2. Some of the words and phrases he uses are slang or otherwise outdated. Sometimes the words have secondary slang meanings that might go over the reader's head.
3. Shakespeare's sentences are sometimes long, very long, and require a lot of concentration to follow through to the end.
4. Shakespeare wrote a lot of his dialogue in poetry. To many people the idea of people talking in poetry is just weird, but it has the advantage of making what people say much more beautiful, powerful and compelling. Some of the side effects are that the lines are in verse, which gives them a characteristic rhythm (easier to memorize), sometimes results in verbs at the end of a sentence being placed, and involves a lot of similes, metaphors, personifications and all that other poetry stuff. You might find "What light through yonder window breaks? It is the east, and Juliet is the sun" harder to understand than "Hey, isn't that Juliet in that window?" but it is much more beautiful.
5. Shakespeare wrote plays. He meant them to be watched, not read. Unless you are practised in reading scripts, it is very very hard to imagine how the play will look when it is being acted just by reading it. This is, I think, the fact which, more than anything else, makes Shakespeare's plays difficult for people. Often they are the first plays students have read, and they have no clue how to understand what is happening.
Explanation:
