Trust, an important quality

My bestfriend is that person in which you could trust 100% at all times with everything, but others do not have the quality of trust towards people nor are they trusted by their peers or even family. A great example is my best friends brother, he is a person who trusts no one and is trusted by no one, he suffers a lot because of this. I spent much time with both of them this Thanksgiving weekend because they are like a family to me and I really got to know his pesonality and its a tough one to say the least. His age is 19, attends college and is a great student. His intelligence is incredible and he is quite nice but for the most part he is serious and close-minded like an old man would normally be. I had many instances with him in which he demonstrate to be a person I shouldn't trust, he would backstab and betray his sister in stupid ways that I thought, what would he get out of that? Anyhow this was just a small anecdote of what a simple trait could do to a person and how you should choose a friend carefuly. Unfortunatly this was a brother and a sister but in other cases its a friend or a cousin. A quick to follo tip is trust and be trusted.