He met ??

Why does Joe speak politely to the man he met ?

"he met" refers to "the man whom Joe met".
The sentence originally is:
Why does Joe speak politely to the man whom he met ?
"whom" is omitted to make the sentence flow better, and is grammatically okay to do so.

Better to be safe than sorry.

It means: "It's better to do something extra to avoid negative consequences, than not do anything and feel sorry when the negative consequences do happen."

Without seeing more of the story, I cannot connect the two sentences, but it seems this sentence may be related to why Joe speaks "politely" to the man he met. He doesn't knwo who this man is, so he plays it safe by speaking to him politely. He is cautious so that he won't need to feel sorry if the man turns out to be his new boss or someone of authority.

001的解釋完全正確。雖然全用英文可能未必讓問者全然明白。

Better to be safe than sorry.
是一種諺語，意思是：小心駛得萬年船

因為當你在一個社交或工作場合剛認識一個陌生人，你不知他的「來頭」時，最「保險」的做法是有禮貌地對待對方。

Why does Joe speak politely tothe man he met ?
(the man he met ) 在這裏= tell you more about the man or who the man is

Better to be safe than sorry.
= 安全做法比日後後悔好
(

to be safe than sorry和 he met不 相同