How to disappear completely? ELI5

How can you speak or text someone almost every day for six months, plan in some depth, relocating to be closer together that entire time, introduce them to your family, plan on having them meet the rest of that family over time, share details damned down to the bowel movement, and After knowing each other for over 20 years Just Disappear...

With one phone call... following the second misunderstanding of basic communication

When you know they're vulnerable When they've told you everything When they've listened intently to you open up, and you don't open up to anyone.

Because they asked for your attention when you were already giving it just moments before.

When they asked you to handle them with so much care

Explain like I'm five years old how you do that?