No man is an island, entire of itself; every man is a piece of the continent, a part of the main. If a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a manor of thy friend's or of thine own were: any man's death diminishes me, because I am involved in mankind, and therefore never send to know for whom the bells tolls; it tolls for thee.
I have always laid it down as a maxim -and found it justified by experience -that a man and a woman make far better friendships than can exist between two of the same sex -but then with the condition that they never have made or are to make love to each other.
It is not merely true that a creed unites men. Nay, a difference of creed unites men - so long as it is a clear difference. A boundary unites. Many a magnanimous Moslem and chivalrous Crusader must have been nearer to each other, because they were both dogmatists, than any two agnostics. "I say God is One," and "I say God is One but also Three," that is the beginning of a good quarrelsome, manly friendship.
If friends disappoint you over and over, that's in large part your own fault. Once someone has shown a tendency to be self-centered, you need to recognize that and take care of yourself; people aren't going to change simply because you want them to.