Why is NOT the possible factor of success according to the man in the conversation?

Lena: There are a lot of things that you can’t control in this world, is that correct?
Ahmed: Exactly. So you just keep going forward. You keep going, endure and perhaps something good will come of it.
Lena: What should be done if you do want success?
Ahmed: You got to work hard, real hard. If you’ re really an expert, maybe something bad won’t come of it.
Lena: Do you think nothing is as good as you want because of other people’s perceptions?
Ahmed: No, I think one may overestimate their abilities. That is the problem.
Lena: What do you mean by that?
Ahmed: Pat it this way, you should never take it for granted your abilities, be whatever they are, they can, alone decide what will come of them.
Lena: Can you give me an example?
Ahmed: Well, for example, in talking about a performance in a film, there are so many other people involved. The performance is pretty much controlled by the editing. You’ re now relying on the editor and the director. If you’ re editing the film. ,you’ re relying on the actor and the support team and on the terrible hand, you might not be doing enough.
Lena: It sounds like, with whatever you do in the world, there are so many factors that you can’t control.
Ahmed: Absolutely.