There's always a reason why people change! In relationships, a sudden change usually means that person has found someone new! That's why they don't talk to you about it! In friendships, rumors can cause the problem.

people change because they are always influenced by their surroundings. There doesn't neccesarily have to be an indententfiable reason for that change, beyond that it is diffrent from how they were. Many a time people don't change they merely wear mask and once the mask comes off you feel as if they changed but thats just how they are.