What is the theme in Lonesome Dove by Larry McMurtry?

Cowboy and husband is a dichotomy that makes up a theme in the book. Augustus McCrae lives his life romantically remembering the one that got away. Clara is the love of his life, but she's not the only love. He also loves gambling, whores, whiskey, and adventure, not necessarily in that order. In the time before the cattle drive, he had the opportunity to make a decision, and likely made the wrong one. When Clara told him she was going to marry Bob, he could have changed her mind. Bob wasn't the love of her life, and Gus was. The only thing that stops him from taking Clara into his arms is his own fear.