Do I have to give a reason for ending the conversation?

Yes, everyone needs to select a reason for why they are ending a conversation. This gives the other person closure, so you can both move on. Nobody likes being ghosted.

Will the other person know why I ended the conversation?

We'll let the other person know when you've ended the chat with a simple closed conversation note to let them know you're moving on, so they can too (but don't worry, we'll keep the full details to ourselves).