Too often, these words are said to cover something hurtful or selfish that was done. Instead of owning up to hurting the other person, someone might say, “You’re so insecure…” or “You took it the wrong way.”
Instead, apologize for the hurt you caused, even if it was unintentional.
If you are saying it because it is indeed true that they are insecure, then this really won’t be helpful. It’s like telling a crying person “everyone breaks their leg at some point, stop being such a wimp.”
(This phrase is also used often by a narcissist as a form of gaslighting or making the other doubt themselves.)