I would probably have quoted some crap from the net. But I have a feeling you don't do Nostalgia. But, you have to find someone or something else. Your rock, does not have to be a person, or even a physical object. It could be a simple thought. The best rock for your life would be an idea. Since, they never leave or die. They simply change and grow. So, calm yourself and go on a small search. You may find something worth going on for. It might not even be what you would have expected.
Now, that part is covered. I will have to probably tell about the change. It may not have anything to do with you. But if he was caring in giving. Than several things could have happened to him. He could have gotten tired of helping everyone. Someone could have taken advantage of him. Which this does happen to a lot of nice people. There is that one jerk, who abuses the kindness that is given until the well goes dry. He could have faced something that could have had a lasting effect.
The thing is, something happened to him. People's personalities do not alter so drastically. Without some powerful outside influence. It has to be strong enough to leave a permanent scar. Even head trauma can alter a person 180 like that. Even if those head traumas are in small amounts. So, don't end up hating him extremely. If anything send him to the neutral pile for now. Because something may have happened.