Was on SFO-EWR midnight flight: was kindly asked to swap seats by a lady who wanted to sit next to her daughter. We exchanged seats that were of equal value to me: E+ window for E+ window. I haven't even settled into my new seat when again I am asked to swap seats so that two friends can sit together. I refused the seat swap into a E- middle seat and this lady just goes off the rails, threatens to be loud and disruptive during flight if I don't swap with the friend. FA came to rescue by stating that if she really wanted to sit by her friend that she would have to give up her E+ for that of someone in E-. Thankfully the FA and I had similar reasoning: seat swap for equal or better seat according to the person being requested.