Why do sellers sell good rental properties?

[Anita Parsa]

I'm looking at a multi family unit which would be a turnkey rental opportunity for me.  I'm a total newbie so I've run the numbers two dozen times with conservative assumptions on everything (high vacancy, high repair costs, etc.) 

All three units are occupied; two through spring and the other is month to month. The building is well maintained with lots of updates and is in a good location.

The numbers look pretty good which makes me if I've missed something important.  As a new investor I'm not confident in myself yet and I wonder if I'm missing something that should concern me.

The seller told my realtor that he's selling because he wants to concentrate more on vacation property around a nearby lake, so maybe he needs the equity from this property to expand his more profitable vacation property?  Is that believable?

Newbie is nervous :)