The validation theory is when person "A" has a habitual habit of falling for another person's "significant other". The tendency actually is a way for person "A" to validate the "significant other's" way of being while in a relationship. The way the "significant other" loves, gives, jokes, reacts... are all validations and also a quick way to find out if "significant other" is worthy material. There are many synonyms for this kind of person... but I believe the "Validation Theory" is an explanation of why they do it - more than a term used for the person.
Katie and John argue a lot... he's such a dick to her. She's so pretty and sweet. She's always doing things for him.. and he said he doesn't even have sex with her anymore... she's the ideal girl. (Justifying to himself things that make her "qualified")... this is validation. The term above is the validation theory (the acts of...)