When you pick up a rifle, or any other object that has seen many previous owners, you wonder about the events and people it has seen. A large part of the draw for historical objects are the stories behind them. This is why, when the history of the object being sold is known and documented, the value can multiply by 100x.
When you are talking about guns, you wonder what the soldier carrying it experienced. When you find blood, shrapnel marks, bullet holes, etc., you know the previous owner went through something heroic, tragic, and life changing. You run through the various stories you have heard, both fiction and real, that might fit the scenario which would have forever changed the gun which you now own.
It is neither morbid nor disrespectful. Rather, it is an appreciation for what that soldier went through. No matter what flag a soldier fights under, every soldier fights for one thing in the end - that is his life. No matter where you are from, you, as a person, can appreciate the thoughts and feelings a soldier has as he carried his weapon while trying to survive the most dramatic ordeal of his life. Those gouges, shrapnel marks, and rust from spilt blood simply mark a moment in time, no doubt the most dramatic in the life of the firearm and possibly the soldier carrying it.
If you cannot appreciate the struggle for which it signifies, I do think you need to live a more fulfilling life.