Losing Your Identity

I've always had a dislike for people (I guess it's more so a girl thing) who lose their identity and personality and everything else that comes with you when they get into a rather unhealthy relationship. Have you ever known one of those people who don't talk to their friends anymore, who don't hang out (unless s/o is busy or out of town) and basically have lost their entire social life? Yeah, you know what I'm talking about... not too appealing huh? It's those people you don't want to be around since you know you're a rebound, yet you pity them cause they have nowhere else to go, so in the end, your sympathy for them wins out at the end. I've always wondered what people find so appealing in losing all their friends (same or opposite gender) just to make a relationship "work"? Honestly, if a relationship is healthy and is supposed to be working just fine, aren't y'all supposed to enjoy the company of you and your s/o in the mist of your friends so everybody wins?