To be honest, I think motivation, or perhaps I should say, the WillPower to succeed, to fight on, to finish thing's, can only ever come from within ourselves.
Sure, we can get advice or picked up when we fall down by other's, perhaps a friend, maybe family, or possibly a complete stranger. But, those are only temporary, relying on others occassionally is perfectly fine, but true motivation, true strength, is only ever truly longlasting when it comes from within.
The reason could be anything, from a grand ambition of worldy proportions, to the most insignificant little thing you could imagine. These thing's dont have to be set in stone, ideals and motivations change as we age and achieve. In the end, it is these things that give us what it is we need to stand up, to fight back, and to persevere. No matter the difficulty we face.
This, is one of humanities noblest ideals, and one that personally allows me to still have faith that there yet remains good in this world. As for myself, my motivation is, and in my heart shall forever be, to bring about the happiness of she whom I love. No matter the personal cost.