A change?

I have been meaning to write about this for a very long time. I see I have the time now to go deep into this topic. Its a very old and basic idea but one which I have a lot of difficulty understanding, although experienced it enough. The topic? A change in personality....
I will start out with some friends as examples. Now a person i know used to be emotional and trusting while being a fun person to be around. This person is still a fun person to be around, very much so in fact, but the former two qualities have just disappeared. I kid you not, this person is now made of stone and the trust issue, I can tell you that I have been a friend for a while now, and I am still to earn some trust. So lets understand how this happened. The claim is that constant fights among friends, betrayals from close friends and a failed relationship are to blame. Sounds familiar doesnt it? But then again is this reason enough to completely change who you are? Because I am sure deep down emotions still run and the want to trust is still there. But I guess I of all people can understand the fear of committment.
Lets take another one. This one is actually pretty sad because I honestly thought I knew this person better than anyone. But over the past few months I have been just shake my head in wonder. I dont think I should go into details in a public blog, but lets just say there was such a dynamic shift that I didnt recognise that person at all anymore. In fact, still dont.
Now I guess as one last example I can take myself. Anyone who met me in my first year here can vouch for the fact that I was a first rate jackass. I humbly accept it. I was a jerk. But second semester onwards there was a maganimous shift in my personality wherein I realised a lot about how to handle people, how to conduct myself while, maintaining my own distinct identity. Now lets go into some detail.
The first thing I feel that motivates a change like this is a deep dissatisfaction for the way life is going on at that moment. To expand this an extremely strong hurtful sentiment, betrayal.. well you get the picture. An event or series of events occur which hurt you very much and as a self defence mechanism the mind moulds itself to prevent a recurrence of the same. Which is why I think that popular phrase, once bitten twice shy.
The second thing that motivates this change is a role model. This probably has happened to all of us at some point. When faced with a crisis we ask ourselves how would this person have handled this? We all have someone we consider a role model. I hence assume they obviously would inspire change.
The third and final thing that motivates this is a catastrophe in life. Like the sudden death of a loved one. Nothing changes a person like grief. I wont go too deep into this.
This change does not happen overnight. It happens over a period of time, sometimes unknowingly. My question to you is this. Do people maintain their individuality at the end of the day? Do people have atleast one person in their life to who they are exactly themselves, not conscious of what they are saying or how they are standing or how they look?
Is such a change healthy? Cause in the previous two examples I think not. In my case it was a positive change ( or so i feel ) hence am continuing in that way. But once someone is hurt by an act, they automatically assume it will recur and never open themselves upto another chance. I feel if you are hurt, heal, but then go back to play. Living behind closed doors will only remind you how few windows you have.....