You have to remember that Sam and Frodo grew up together in a small community. They were like brothers to one another. And the brotherly love they had for eachother was brought out to the open because they were going through such hardships.

Hardships like they were going through will bring people who truly care for eachother closer together. And will drive people who don't truly care for eachother further apart.

Sam and Frodo probably wouldn't have acted like that if they were just living normal lives. But because they were going through such difficult times the brotherly love they had for eachother was brought out in the open.