I think that everything that happens is on purpose. Every person we meet is in our lives serves a purpose to enhance our human experience.
Sometimes the person may not seem to serve a direct purpose for you, but maybe you're there to serve a purpose for them. There will be friends that make enormous contributions to your life, while others not so much, but perhaps your presence is essential to his/her life.
I belive that the only coincidence in life is the fact that certain events, people, and experiences coincide with your life, when the timing is perfect.