I play poker with some of my closest friends a lot

many months ago i played a 1v1 game with one of them

I had a weak hand
but i decided to bluf by raising the bet gradually
he saw right through it, i know because when he took the pot he told me that i made it obvious
this means that he thought that such behaviour ALWAYS means that a person is bluffing

anyway months later a group of us were playing and i notice that he was showing the same behavior except that he had gone all in in order to follow the strategy.
To me this was proof that he was bluffing because he had previously told me that he thought that i was bluffing while i was doing the same thing.

unfortunatly for me i had only a pair of 7s
so knew for certain that he was bluffing but i only had a mid level pair so he could have beaten me with whatever he had even if it was nothing more than pair of 8s

I decided to call him beause i would rather loose lots of chips than let him play me for a fool

and i won

what would you have done in the same situation?

It's called a tell. Any behavior exhibited by a player that reveals the perceived value of their hand. I say perceived, because some players don't understand how their hand stacks up.

Imagine

with a board of

That two pair looks pretty crappy, but to a player who can't read the board well, all they're thinking is "I HAVE TWO PAIR!!! WOOHOOO! I GOT TWO PAIR ON THE FLOP!! I IZ LEET!"

Reading and capitalizing on tells is just part of the game. If you and the friend are real close and are intent on improving your games together, then tell him of his tell, and he should do the same for you. But if your just social players, and anything really goes, then hang onto that tell for a while.