It may seem odd and frustrating to you. Are they jealous? Self-conscious? What is it? Most friends, (if they're good ones,) will look out for you no matter what and will try to do what's in your best interest. Sometimes that entails doing or saying things that may not make you happy or that you appreciate.
But good friends will always speak up and let you know if they see you harming yourself in any way. That's why seeing a friend in a destructive relationship is one of the hardest situations to handle. Most people in bad relationships refuse to see the problems, and if they do, they brush them off, thinking they're normal for any relationship. Your friends know better than that, and when they bring it up, it's their way of telling you that you need to get out of the relationship and save your dignity.
Imagine how it feels for the friend—they know that no matter what they say, you're going to defend your relationship and get upset with them. They know they may even jeopardize their friendship with you. However, if they keep quiet, they'll go along with it and refuse to acknowledge the destructive behavior that's going on. It's a predicament that's tough, but that any good friend should know the answer to. If you see something suspicious or detrimental, you bring it up.
The way that you respond to these criticisms is really key, though. Being open-minded, listening to what they have to say, and really taking into consideration their honest concern for you is the right way to approach things. Understand that it may not have been easy to come to you with their qualms, and they just want to see you healthy and happy. Take a look at what they are criticizing and see if they have a valid point.
Are you being put down in any way? Could your relationship be harmful towards your mental health? Are you pulling away from others in your life? These are some of the most common things that friends see in bad relationships and worry about. Be respectful, and if you feel they are out of line, let them know that while your relationship isn't perfect, you're going to work on it and try to make it what it should be.
Occasionally, a friend will come to you and point something out that you don't have a problem with, or that you think is not a big deal at all. If this is the case, take a look at your friend's possible motives. Would they have any reason for bringing this up? Could they benefit from it in any way? If they would, you also need to put that into perspective. A good friend should be there for you no matter what.
And most of them will be—for at least a while. There will be a point, however, when enough is enough and they will just give up on the friendship. No good friend can stand back and watch while their friend puts themselves through a bad relationship and keeps going back. They will get tired of seeing you hurt and in pain and will inevitably drift away. You can stop all this from happening by taking a long look at what it is that they're talking about and ask yourself if there really is something wrong.