People tend to cover up their true feelings behind the phrase, and really avoid discussing their problems who care about them a lot. When you say "I'm fine" people tend to get the impression that you aren't being honest about your feelings One of the biggest lies told in modern times is "I'm fine" because it's sounds more like a red flag and an excuse to just get out of a bad situation.

Read more at: http://www.wegist.com/2016/07/why-saying-phrase-im-fine-means-youre.html

How are we sure the person asking how are you truly cares about how we feel.
Replying I'm fine to me means as far as you're concerned I'm OK, there's no point relating my problem to you.

It means you're asking too many questions