这是课文节选的《百万英镑》，有这样的对话：

1. A: I went to the American consulate to seek help, but... Anyway, I didn't dare to try again.

B: Well, you mustn't worry about that. It's an advantage.

2. A: If this is your idea of some kind of joke, I don't think it's very funny. Now if you'll excuse me, I ought to be on my way.

B：Please don't go. You mustn't think we don't care about you. 

请问这里两处的mustn't如何理解？字典里也有和2句类似的：You mustn't say things like that!（千万别……），请问1处的也是“千万别”的意思吗，还是don't need to 的含义？

因为以前只知道mustn't有禁止的含义，请问如何区别它不同情况的用法？