Dogs are very social animals, some even more than others. Which makes it so that being left alone can be very challenging at times, especially as young pups.
The 2 most common reasons behind dogs who experience separation anxiety revolve around them being left for longer than they've been capable or handling as well as being left alone, for short or long periods of time, with pent up energy.
This means that if you re-introduce being left alone by working on short periods as well as when they feel the most relaxed and mellow (thanks to healthy outlets to not only their physical but mental energy needs as well) you will then have a better chance at having a dog that will not only be able to handle being alone but also partly enjoy getting to have some alone time to rest!