I completely disagree with this, since it implies that disabled people should have to maintain a positive attitude, no matter how stressful their life is, and that's just pure bullshit. If a disabled person is having a bad day, they should be allowed to be negative. Seriously, how can you think this saying makes any sense at all?
What he is saying is that we have the choice to let the negative control us, or not. Crying and complaining don't get us anywhere. It doesn't mean it doesn't have to hurt, no it may hurt so badly, but it is working on moving forward that makes a difference.
So we should never cry or complain, in your opinion? And we should feel bad when we do cry and complain, since we're not doing anything to move forward?