Because they're selfish....and you're lonely ? That happened to me once but it caused me lots of pain when they went again.
Here's a hug from me xoxox xxx
Yes, because you let them.
I do too, but usually when someone comes around again and I talk with them like nothing ever happened... usually that person really is just a random friend rather than a true friend and I won't mind if they come and go.
Now if that person who I call a real friend walks out of my life... that's a different story.
Because you let them.