While wood doors are luxurious and beautiful, they also offer a fair amount of upkeep. Wood is sensitive to moisture, and wetness in the wood can cause the door to warp, crack and change shape. Sun is also an important factor in the life of your wood door. If your door is protected from these elements, it's bound to last much longer.
When shopping for pre-finished wood doors, look for durable stains and high-gloss finishes, since these will best protect wood. If you've decided to apply the finish or stain on your own, make sure to put the finish on the top and bottom edges of the door as well to help prevent it from absorbing moisture.