Cloudy past, mistrust, their definition of love maybe skewed...
Because the other person did not love the same way you did.
Maybe they have never known the kind of love you have saying you would do anything are words not many people say and really mean them.
I don't know as I'm that person who doesn't understand when a person loves me that much..
Sometimes it really is not about you and how deeply you love them most often it's about the fact they feel that no one could ever love them that much as you do ....see it can be about how they feel about themselves.
Unrequited love? Maybe it seems weird someone would love them so much. Probably because they don't love themselves or don't respect themselves enough.