I feel the same completely. I don't believe in friendship at all because of the same reason - they're going to leave eventually.

However, I'm starting to realize that friendships are made on value added basis. For example, when I look back at all the friends that I've had in my life, it's been because I've helped them or received help with one thing or another. It's like an adventure. Once that adventure finished the friendship ended.

For example, I was volunteering in university and made a ton of friends there. After I quit volunteering to focus on studies, no one talked to me anymore. The adventure had ended and so the friendship.

I've come to realize that humanity is a concept of Human Resources. When we use people or when people use us, it becomes a binding relationship. Just like when you get hired at a job, you are a human resource - you are being used to complete a set of tasks that benefits the company.