Why would you tell a girl you found her weakness (tickling)?

I'm extremely ticklish, so I jump and scream when people tickle me, especially if I don't see it coming. A guy at work (who flirts with everyone) tickled me and said "Now I know your weakness." What does that mean? My ex said the same exact thing about me being ticklish
Why would you tell a girl you found her weakness (tickling)?
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