For the most part, the members of the Joad family seem resigned to their fate in regards to their feelings toward leaving home and moving to California. None of them particularly want to leave, but they have watched their opportunities and their livelihoods dry up with their crops and be carried away with the dirt on the wind. They don't want to leave, but life on their farm in no longer sustainable.
The exceptions to this idea are Muley Graves and Grampa; both refuse to leave. Their reasons are very similar. Everything they have and everything they are was devoted to making their farms prosperous; the land is "in their blood." They are too invested and too emotionally intertwined to leave. Muley, in particular, seems to understand that staying will likely mean physical starvation, but he consciously chooses this over the emotional and spiritual starvation that would occur if he leaves.
Grampa is much the same, however, he isn't given the option to choose for himself. With a heavy dose of cough medicine, the Joad family drugs him and takes him away against his will. Grampa's fate, however, isn't changed. Once removed from his farm, he withers and passes quite quickly.
The Joad family was like the many poor farmers of the drought regions during the depression. Each family member had watched their crops dry up and die, the land turn to sand and dust, and had their lives abruptly changed. For the Joads the need to move was as necessary as it had been for the other farmers. They watched as their neighbors had packed up and moved away. The region they had lived in had changed and become an un-inhabitable environment.
For the most part, despite the tragedy of their lives, the family members were ready to move on. They had nothing left for them at their home. They purchased their vehicle with the excitement and hopes of having a new life lay before them. Ma Joad was the guidance that projected hope into her family. They had no idea that their exodus and entrance into California would be met with negativity.