In the great metaphor of marathon running to life, you often end up running the hardest parts on your own.
You have to plod one foot in front of the other until the finish line, because stopping is not an option. The joy of 'finishing strong' is still there, but there is also an emptiness of having no one to share it with.
Napoleon said something like: Courage is not having the strength to go on, but going on when you have no strength.
It's nice, though, to have someone to en-courage you.