Do you remember reading a Classic for the first time and having to trudge your way through the vocabulary and sentence structure?
Like most people, I read some classics in high school. Like most people, I didn't enjoy them as much as I could have. But then time passes, and wonder of wonders: I somehow got used to reading the more complex language that is often found in those older books. The first time I read Pride and Prejudice it took some work. Then I watched the film, and amazingly, the language began to make sense. So much has to do with your familiarity, doesn't it? If you know the story, the language is easier to absorb; if you know the language, the story is easier to absorb.
Here's an entertaining example:
It really isn't that the classics are always so much more difficult, it's that they are often unfamiliar territory. (True, they do stir up the grey matter a bit more, but that's a good thing isn't it? Move those brains!) This is why, if you are just getting started with the Classics, it can be immensely beneficial to pick something with more accessible writing (Gone With the Wind, The Good Earth, To Kill a Mockingbird) or with a story you are familiar with (A Christmas Carol, Adventures of Sherlock Holmes, Peter Pan). This is also why I have no problem watching an adaptation first--get familiar with the story line, and then jump in and enjoy the story-telling.
Once you get past that first hurdle, you may be surprised to find that reading the classics is more a matter of taste than difficulty.