Are ostriches upset that they cannot fly?
When you think about birds, they are very closely associated with flying. A quick sentence verification task or a semantic hierarchy can show you just how quickly your mind associated the word "bird" with "fly".
We all know that ostriches are a bird but they, unfortunately, are one of the few that cannot fly. With their oversized bodies and long lanky legs, their little wings are not capable of lifting them off of the ground.
They probably look around at all of the other birds and think "why can't I do that?".