When I first started to write, it bothered me a great deal that my characters had trouble getting to know each other. Finally, I began to realize that at the beginning of the book my characters were still working out who they were and their relationships to others. Just like in the beginning of a new friendship, you act one way to a new friend because you only understand superficially who that person is. Once the friendship grows, subtle and not-so-subtle shifts occur in how you interact. So by the end of the book, it’s logical that your characters are interacting more naturally.
My advice? Re-write the beginning of your book as many times as needed. Eventually, you’ll feel it flow as naturally as the latter portions.