Could be the stress they deal with at work. Being away from each other is definitely hard. You have to communicate or it won't work. I have been in the Army for 18 yrs and have been married for 17.
That person is gone for days/weeks/ months and you worry about their safety all the time. They also miss so many important things. I guess that's why :):)
Because they are gone a lot and don't make much money
Relationships are two people being together. It's difficult to be together when you aren't together.