It’s a defense mechanism to reject someone you’re feeling rejected by. But if you’re feeling rejected by the person you’re in a relationship with you need to resist that instinct and instead communicate how you’re feeling. You need to find out what you don’t know before you create a problem that might not exist.
You tend to take everything personally when you’re falling for someone. You’re hyper focus on everything your new lover does trying to figure out how s/he truly feels about you. So if s/he pulls back a little, it makes you really insecure that you did something wrong, or that his feelings for have changed. Which in turn usually makes you pull back too—to protect yourself.
But it may not have anything to do with you. You may be getting yourself all worked up over nothing. He could be worried about work, family, his health or finances, all potentially stressful topics he may not be ready to share with you. Therefore, if you withdraw just when he needs your understanding and support you become another problem for him. When all he needed was just a little space to sort out his feelings.
Make sure you’re not deciding for him what he’s feeling. You’re not a mind reader. The behavior you ‘re reacting to may feel like an instinct but it could just be your own insecurity. Unless he has specifically told you how he’s feeling than you do not actually know. If you choose to believe something negative and you’re wrong (which is very possible) you will be the one to destroy your relationship unnecessarily. You need to find out what is actually happening and only respond to facts.
Don’t blame or accuse him or her of anything. Stay calm and empowered by using “I statements” to express your concern. Ask, “I feel like you’ve been a little distant is everything ok?” Give her the chance to calm your insecurities or potentially fix any problems with your relationship.
If it does turn out to be that her feelings for you have changed you’re better off hearing it from her and ending it amicably. However it turns out you will feel much better by getting answers than by making up all sorts of false scenarios in your head.