This seems like a difficult subject. I know I've experienced it a lot in my life, the life of friends as well as family.
As a kid, adults warn us not to talk to strangers because of the whole "stranger danger" thing. The truth is, though, if we don't talk to strangers, we live life alone. The difficult part is to filter who we talk to, who we trust and especially, we need to control our own selves. Most times, people open up to people because the person is extremely nice and seems genuine. However, a lot of times, those people aren't trying to become your friend or they may become your friend, but they don't treasure the friendship you give as much as you might.
In my own life, I've come across many people that I liked. For instance, there was a girl in my life for six years. I cherished her so much and gave my life and love to her. However, she kept pulling away. She treated me like crap. The exact situation happened with almost every one of my close friends. They became so entranced by a person and their kindness or beauty. Quickly, the truth would surface. Either there was cheating, or denial, or guilt trip. Then my sister, became very involved with a guy she had met through uncertain means. Even though they spent every waking moment connecting, when the time came, the guy lashed out and pulled quickly away from the relationship.
One of the biggest dangers of this trust-fail we suffer, is that the person we care for will pull away, or hurt us deeply. We become emotionally unstable and they come running back to try to "repair" the issue. We are extremely vulnerable and tend to welcome the person in with open arms. The problem with this is that it is like we bit into an apple and found out it is poisoned, but we desire that taste so we go back and take another bite, which only hurts us.
This world is full of opposing challenges. To trust people, but not trust them. "Don't talk to strangers" but "get to know people". We must sacrifice something, either our self-esteem and confidence or our ability to move forward and gain experience. Life happens with pain and suffering and we cause a large portion of that pain to each other.