diff --git a/lib/shared/messages/reaction-message-spec.js b/lib/shared/messages/reaction-message-spec.js
--- a/lib/shared/messages/reaction-message-spec.js
+++ b/lib/shared/messages/reaction-message-spec.js
@@ -56,11 +56,15 @@
   },
 
   messageTitle({ messageInfo }: MessageTitleParam<ReactionMessageInfo>) {
+    // messageTitle is used in MessagePreview and for determining default
+    // sidebar thread names. You can't create a sidebar from a reaction, and
+    // showInMessagePreview below only returns true if the user is reacting to
+    // a message from the viewer. So we're safe to use "your message" here.
     const creator = ET.user({ userInfo: messageInfo.creator });
     const preview =
       messageInfo.action === 'add_reaction'
-        ? `reacted ${messageInfo.reaction} to a message`
-        : 'unreacted to a message';
+        ? `reacted ${messageInfo.reaction} to your message`
+        : 'unreacted to your message';
     return ET`${creator} ${preview}`;
   },