I think the one thing that kills people in life, and stops them from achieving their biggest desires; is satisfaction. Being plainly satisfied with something, gives us no drive. You become much too scared to make a move towards the better, because you are scared of losing everything that had you "just satisfied" in the first place. And you aren't as happy as you could be, because you are merely at a norm. Norm is not good. Norm is the standard. Being below the standard makes you aim higher, and being up high makes you never want to lose that spot at all.

Satisfaction defeats Drive.
Lack of Drive defeats Potential.
Lack of Potential defeats Happiness.
Lack of Happiness defeats you.