According to one of those fact things on Twitter the average person tells 4 lies a day or 1460 lies a a year; a total of 87,600 by the age of 60. And the most common lie is: 'I'm fine'.
We've all done it, some more than others. Sometimes it's because we don't want to discuss what's going on in our lives, sometimes it's because we don't think anyone else wants to hear what's wrong, sometimes we just say it because it's the first thing that comes to mind.
I probably say it at least 6 times a day regardless of how I am actually feeling. Those people you know you will know if you are fine or not and will ask how you are regardless and won't stop with the 'I'm fine' response.
There are those of us who actually don't want to talk about it though, because sometimes discussing things doesn't actually solve anything, it's a personal journey, something only you can resolve in your own way. But knowing that people do care, that there are friends who would take the time to listen and advise, is a comfort, even if you don't utilise them.