In my opinion, over-dedication often is linked with low self-esteem on the part of one member of the relationship. They don’t feel they’re strong enough on their own, so they dedicate themselves entirely to their partner. The mentality behind that is generally “I can’t be happy unless my partner is happy.” Fear of losing that person for not being dedicated enough to their relationship also relates to this.
Dedication is important, it shows you take the relationship seriously. But putting one’s relationship (and their partner) higher in importance than one’s own health is dangerous and often leads to unhealthy relationships.
I find the best way to avoid this is to spend enough time being single to learn about yourself and what makes you happy. That way, you don’t feel that you have to devote yourself to your partner in order to be fulfilled.