This isn't anything new. I read Harry Potter and the Deathly Hallows when it came out 4 years ago. And loved it. The movies finally ending this year has made me feel nostalgic. I've reread the Harry Potter series at least 20 times. It's always as good as it was the last time. Sometimes even better, when I understand something better than I did all those years ago.
That time 4 years ago, I didn't really say goodbye to any of those characters, I didn't feel as sad as I might have. But now...it's been a while since I've read them. Ever since the advertising for Harry Potter and the Deathly Hallows Part 2 has started, all those pictures and quotes on tumblr showed up and really got to me. I remembered how amazing reading the books felt like. Like it was all real. And everyone was with me, Harry, Ron, Hermione and everyone else. Like they were my family or friends. I remember starting to cry when I read that Fred died. When I read a book, I get absorbed into it, and I'm with them in their world, at Hogwarts.
Now I can go on and on about Harry Potter, but I'll stop here, and leave some for later when I go and watch it :)
Anyone who hasn't read Harry Potter - go read it NOW. I don't care how old you are, if you've seen all the movies or whatever. Just go read it, and fall in love with almost all of the characters.