"No one will ever love you as much as I do."
The explanation given for this phrase, "I want them to know how much I love them!"
Here are the reasons it isn't effective:
It doesn't say, "You're special." It says, "I'm special." It's about ME!
It suggests, "You are unlovable if it weren't for my unique ability to love you."
It communicates, "I need to keep you dependent on me. Unless you need me, I don't think you'd stay with me."
Unfortunately, many victims of abuse in childhood have these words spoken to them. Many adults hear it from their significant other. It's especially confusing because the speaker often has deeper insecurities than the person they are trying to make dependent on them.
The person may really love you; their confusion doesn't make them bad. They are more likely worried about your love for them.
See it for what it is.