So many scenarios are possible. I know that people don't want to seem to accept the fact that he *could* have walked away to a new life, but I guess that might be because his wife doesn't believe that. In her having him declared dead, she listed the reasons why. I still think suicide is a possible scenario but I know many won't believe that. The thing is, you never know what the true state of someone's mind is.
Some have suggested a robbery gone bad, but many don't think that true, because the lack of missing things and the fact that he is not found. Robberies gone bad usually end with the victim dead and present. Plus what robber locks up behind them, after taking nothing?
Kidnapping? No ransom request makes that unlikely.
A romantic rival? Possibly. Would someone kill him to be with his wife? It's happened before.
Walked off with amnesia? Could be. It's happened before.
One thing I always keep in mind is the saying, "When you hear hoofbeats, think horses not zebras."
What is the most logical scenario, when you take into consideration all that we know?
If you're making a chart with all of the possibilities (that we can think of) and putting facts under each 'theory' which theory has the most points in its column?