From my observations, most people who return to drinking after an extended period do it because they get lax or give up completely on their recovery because they think they are "better". I firmly believe that as long as you follow a plan you can almost guarantee continued sobriety as long as you choose to do so. Later in sobriety of course you might not need to spend nearly as much time every day on it, but i think it's important to at least acknowledge it daily...even if it's just reading some posts here or doing some reflection on your own.
We do all have days where we are "down" and the future is unknown for any of us...but we can control our daily activities. And keeping vigilant every day can eliminate some of the future bad possibilities.