The book The Complete Idiot’s Guide to the Akashic Record: Tap into the Timeless Wisdom of the Universe (Complete Idiot’s Guides (Lifestyle Paperback)) talks about Soul Mates and Soul Contracts, and how they are together for a reason.

This book has some surprisingly useful information in it, despite the title!  It covers Soul Mates being together under the following circumstances:-

Soul Mates
Twin Souls
Unrequited Love
Difficult Relationships
Lack of Relationship
Family Ties
Parents, Siblings, and Children.

Soul Contracts are pre-life agreements that result in the high likelihood of people meeting or even reincarnating together.  The reason that Souls connect through Soul Contracts is to work out karma and to grow as individual souls.