As with any kind of human interaction, it all boils down to how you see yourself. If you start to appreciate yourself and focus on your positive qualities, you will send out a vibration which attracts people who really appreciate you: by definition, friends. Very often, introverted people focus on their shortcomings, and how they think people will dislike them, but in truth, most people in the world are good natured, and appreciate the people they like for being who they are. Most extroverted people who have a number of friends, will be friendly with all different kinds of personality types, because they like the variety. This will include some introverts too. If you send out the vibe of self-appreciation, you will be giving the friendly people of the world something to tune into.