Interpersonal relationships develop for many reasons. If you usually do not stand up for yourself, that can be an attractive personality trait to someone who likes to control another. The relationship works for them.
When you ‘stand up for yourself’ you are changing the rules of the game. Sure, its their game, but for the most part, you are a willing participant in the game.
Other answers to this question have touched upon the concept of assertiveness however haven’t expanded upon it.
Simply put … assertiveness is getting what you want out of life, without it being at the expense of the other person. This is called a ‘win-win’ situation.
I’m making an assumption here but when you say “when I stand up for myself” it sounds adversarial. Assertiveness can be placed on a continuum. On one end, we have being passive, and we are assuming that you are until you aren’t. The other end of the continuum is where people are aggressive. Being assertive lays somewhere in the middle.
Some people believe that being aggressive is like bringing out the super guns. The best defence is an attack. “The bigger they are … the harder they fall!” That’s the way I was until I learned about being assertive.
Your question asks for the why. When you stand up for yourself, you may be threatening to the others. In addition to the reasons cited above, they may be insecure about themselves, you may be scaring them i.e. they could be afraid of losing your relationship, or they could jut be plain angry.
I often use a story in my clinical practice that I learned years ago “If the only tool you have in your toolbox is a hammer, then every problem you have will be a nail.” If standing up for yourself means lashing out in anger, then perhaps its time to develop some more tools.
Learning everything you can about assertiveness and putting it into practice can go a long way in improving your self-confidence but changing your interpersonal relationships into effective, healthy ones.
Thanks for your question and good luck with asserting yourself!