Some people go into new friendships with a defeatist attitude, unconsciously giving off a vibe that says, "stay away." If you look for the times that friends disappoint you, you'll find them. This approach will become clear to anyone who even tries to get close to you. As a result, you'll have a harder time making friends.
Realize that people are human, they make mistakes, and they probably will mess up from time to time. But rather than backing off friendship altogether, learn to see the good in people and trust that when someone wants to be your friend, they mean it.
This is just one reason it might be difficult for you to make friends. Here are five more.