The prefix com- acts as a strengthener in this word; thus, to compel is to drive powerfully, or force. So you may feel compelled to speak to a friend about his drinking, or compelled to reveal a secret in order to prevent something from happening. A compulsion is usually a powerful inner urge; a compulsive shopper or a compulsive gambler usually can't hold onto money for long. You might not want to do something unless there's a compelling reason; however, a compelling film is simply one that seems serious and important.