Trust is very important in a friendship
Most of them times when we have been betrayed it is hard to trust people. It is especially hard when we have been betrayed by a close friend or someone who we trusted highly.
We go around trying to be ourselves and be friendly. The minute we open up to someone we are scared they are going to hurt us.
It is important to be on guard but it is also importnat to trust. If you have a friend who you can't trust then you will always be unhappy when you are around them. You will fear the worst and it will cause conflict between the two of you.
Get to know someone over time before truely opening up to them. Don't tell them a huge secret right away. Don't brag but if you see that they are happy when something good happens to you then you know that they could be trusted.