what is why not in Japanese
 

like
"why u always hanging out with those guys?"
"why not?"

If a speaker is a girl, it would be;
いいじゃん
いいじゃないのよ
悪い？

If a speaker is a boy, it would be;
いいじゃないか
いいじゃんかよ
悪いかい?

If a speaker is a boy in Kansai-area, It would be;
あかんか？

別に　maybe? if that's not too strong
or
ええやん in kansai dialect ^^b

or combo it,
ええやん別に -> why not, that's my business

でしょ？日本人の方？

got that, thank u very much.

はい、さようでございます。:D