Harry Potter. I tried reading the first book and found it intensely dull, causing me to basically give up on the books, though I did watch the first few movies. Then I met my partner, who loves the books, and she got me to try reading the third book, which I read in 2 sittings or so.
Ulysses. The first time I read it, I was very focused on finishing it and making sure I sounded smart when I talked about it. The second time, I was lucky enough to have an instructor who encouraged us to not worry about trying to entirety of the text, but rather to just try to find one interesting thing about it and focus on that. I ended up staying up til 4 AM several nights in a row, trying to figure out the text's approach to ethics. I still don't understand the text's approach to ethics, but it's funny how much my enjoyment of the text changed when the "weight" of the novel was removed and I was just able to focus on something I found interesting.