Not everytime，对……感兴趣 just like be interested in sth，so that means if you just want to say：Yep I feel interested. in Chinese you can say：是的，我很有兴趣 or 对的，我很感兴趣，you don't need to add 对。 But if you want to say：I'm interested in sth，there's a subject you want to talk about，you need to add 对。我对学中文很感兴趣，I'm very interested in learning Chinese.

hi.i am kim.can you help me i have questions.:)

Hi Mako, not necessarily. Here are some examples: 1. 这是一部科幻电影，你怎么会感兴趣？ 2. 这个摄影师让我很感兴趣。 3. 他很幸运地找到了自己感兴趣的工作。 4. 我感兴趣的是：这道菜是怎么做的。

It's necessary only when there is an object. 对+object+感兴趣 Otherwise it's not necessary. 我最感兴趣的事

Yes