How many times have you asked someone how they are and they answer with the single word "fine"? How many times have you answered the question with just "fine"? You'd probably lose count if you tried to count the number of times. It's not unusual to use this one word to describe how you are, but is it the best answer you can give?
When someone asks the question "how are you?" and we answer with "fine" we are being extremely vague... and probably untruthful. Plenty of people use "fine" to describe how they are. I use "fine" to describe how I am. But there are so many other adjectives that we can use to describe how we are feeling.
We could say our day has been: horrible, magnificent, interesting, weird, disastrous, wonderful, the best, the worst, pretty decent, normal, amazing, etc. We often just say we're fine because it's easier than saying exactly how we feel or how our day has been.
And what do these more interesting adjectives do? They're more truthful, and they start more of a conversation than "fine". If someone says their day was weird, wouldn't you want to ask why it was weird? You could get a wonderfully interesting story about how their dog can speak 15 different languages. Or you could get a wildly intriguing story about how their kids bought a horse with the help of their knew babysitter. We can do much better than "fine".
Most of the time, if we had a bad day and told someone we were "fine", it's because we don't want to bother them with the details of us stepping in gum, falling down a flight of stairs, getting yelled at by a customer while we're at work, etc.
However, some people say they're "fine" even if they had an amazing day. We could do this because we simply don't want to go into details of our day, or because we feel sorry for the person we're talking to. Maybe the person we're talking to just finished telling us about their car that got broken into, their fish that passed away, or their phone that shattered.
We should talk about our days, be social, help one another through their tough days, and celebrate one another's happy days.