What is Friends?

I met a best friends since 2010
same class with him.

But he give up to continue to study in 2011.
when he left the school
he still my best friend.

but now , he had been change to a stranger for me

2010
He teach me how to play DotA
until same level with him
Solo 1 vs 1 with me , didn't bully me but just teach me

when i talk something wrong , he will guide me to the right way that how to talk and peoples will not angry.
thats why I have many friends now and i know that how to make them happy.

2011
we still invite him come out with us , he is too busy because he needs to work.
still a kind friends , or like a big brother for me.

2012
Ok all Change , we grows up , we are 18 now
he change too! 20 years old , he didn't use the way that he teach me to solve some quarrel in friends group.
he using another ways such as gangster to solve problem?
why?

『
You had change from a good friends to a stranger
From teach me Dota , How to solve problem nicely
until now you solve problem with some bad ways.
』