Building a future of friendship between humans and bots

How do you actually become friends with someone? Is it a slow process of getting to know each other or is there a moment when you just hit it off? Do we enjoy staying friends with people who have become part of our lives or does there come a point when we just let them go? I was Facebook friends with a woman named Claire when I was in college and we used to discuss sci-fi movies together. Though we've lost touch, I miss the friendship we had developed over that very specific topic of conversation.