In the heat of a negotiation, you may find significant resistance from your employer (or potential employer) who may not have been expecting you to assertively advocate for yourself.
If you feel tensions rising, or get an unexpected response, consider asking questions that will provide you key information and keep you in the strongest negotiation position possible. When you learn more about where the other side is coming from, you can use that information to strategically respond in ways where you both benefit from the outcome.
Consider something like "You seem surprised/off-put/etc. about the request I just made. Could you tell me a little more about that?"
"While my take home salary is the most important to me right now, I am open to continuing this negotiation. Could you talk about what parts of the compensation package might be most appropriate to discuss?