When I say you can never know a person well enough, I mean not just people around you, but including yourself too. 

Sometimes you hear people saying,

"I won't do this."

"He won't do this."

"She won't do this."

"Nah, it's not me."

"I just cannot imagine him/her doing this."

"That cannot be him/her."

and the list goes on...

But the fact is that NOBODY can be extremely sure that that certain person or himself/herself will not do certain things or will not act a certain way. They think they won't but they might.

This may or may not have came across your mind. How we act, what we do or what we say differs from the different circumstances that we are being put in. If you are put into a situation which you have never came across before, the reaction you make may come off as a surprise to you.
To the extent where you may ask yourself,
"who am I?"
"Is this me?"
"What am I doing?"

I can say even if you're not at school learning, you're still learning. Learning who you are, learning who the people around you are. They may not be who they said they are, you may not be who you think you are.

I find myself changing after knowing different kinds of people. I found a personality I never thought I would have a few days ago. You know what would make a good people cycle?
So I say, you need to be prepared for unexpected personalities, changes, reactions from people around you and YOURSELF.

Don't be so sure who you are yet. Don't be so sure who the other person is yet. Because you can never know a person well enough.