Why is the underlined selection NOT necessarily a good argument?

The first thing to do is to interpret what Mark Antony is stating here. He is saying that he presented Caesar with a crown three times at the Lupercal feast. Three times ("thrice") Caesar apparently refused to take it. Mark Antony is implying that this shows that he was not ambitious. However, it could well have been the case that he did not take the crown precisely because he wanted to seem unambitious so that he could appeal to the people. Mark Antony's argument here is charged with rhetoric, but that does not diminish its weakness. We must be aware of such uses of language so as not to be tricked. Matters just cannot be seen as being that simple.