You need to find a balance. Don't let people in all the time, but don't shut them out either. You gotta get to know people so you can figure out who to trust and who not to trust.
People will always hurt us no matter what. The reason being we are sensitive, even if we don't want to tell ourselves that. We are human we have been disappointed and hurt before so that wound is always open even if we do the "out of mind, out of thought" or "close the door to the experience". It is that past hurt that we never healed from that makes it hard to let anyone in. We simply do not want to hurt again. I think we have to take chances but at your own pace because NO ONE but you knows your circumstance, your hurt. Patience, kindness are NOT weak virtues..use these to help you. YOU ARE NOT ALONE in this.
It is better to let people in while being cautious and knowing you might possibly get hurt but it is worth it in the end.
Sometimes being quiet is detrimental too. It depends on situations. Sometimes you have to speak up and sometimes observe. :)
I think the saying goes "better to have loved and lost than to have not loved at all"
Probably better to let people in, if one has something to let them into