「佐藤さんも入らない？」 vs 「佐藤さんも入ろうか。」


> 女性：田中君。テニス部に入ったのね。面白い？
>
> 男性：  
>  a - うん、佐藤さんも入らない？  
>  b - うん、佐藤さんも入ろうか。

I wanted to know why 'a' is correct and 'b' is incorrect. The woman is saying
"so you entered a tennis club, is it interesting?" Then the guy answers "sure
why don't you enter also with me", so that's why I thought 'b' would be
correct, why is 'b' incorrect and 'a' correct?




In this case, the man is already in the tennis club, so 佐藤さん is the only
person who may join the club. When よう works as a suggestion that only involves
the second-person (i.e., "why don't **you** " rather than "why don't **we**
"), it sounds strong and obligatory. It's perhaps like "You'd better join,
too" or "(I know / Of course) you're gonna have to join, too". This type of
～ようか is used only if you have a good reason to have some control over the
other person's thoughts and actions. 佐藤さんも入ろうか may even be taken as a
harassment depending on the relationship of the two.

On the other hand, "入らない?" is a safe and casual suggestion (just asking what
the woman thinks about it).

