Why it matters
Let's discuss something generally avoided in professional settings - your feelings and emotions. Yep, those scary things.
You know that feeling of nervousness you get in the pit of your stomach when you're having a hard conversation? Or the sweet relief you feel when it's over? Turns out those emotions matter a lot. I like to think of negotiations as existing on an emotional continuum from sweet to sour. As a rough rule of thumb, sweet is driven by compliments ("You're the best...", "We need you...") and smiling. By contrast, sour is usually driven by threats ("I can't believe you're...", "We may need to pull the offer if..."), silence, and scowling.
Emotions matter just as much as words. Stating "The offer isn't competitive!" in a squeaky, upbeat voice is not going to be convincing. Nor is saying, "I want to work here!" while yelling and scowling. When you speak, let your emotions match your words.
I've generally noticed that offerletter.io clients in Product Management, Sales, and Bizdev, have the best intuitive grasp of sweet and sour emotions. Data Scientists and Engineers can lack the intuition required by those other roles, and so tend to need the most coaching.
It's generally best for you, the jobseeker, to be a little more sweet before the offer arrives, and a little more sour when you're negotiating the offer itself. Even if you're new at this and can't drive the sweetness/sourness of a conversation, be aware that your emotions can and will be driven, especially by unscrupulous recruiters. Above all else -- stay calm, and don't feel rushed when negotiating. This is precisely why managing the timeline, again, is so important.