Let's do some thought experiments.
Imagine you just started dating someone who likes to watch scary movies. You absolutely can't handle them. Do you agree to try to get through it? Even worse, do you pretend to be excited about it, even though you know you'll be sleeping with all the lights on for a week (if you sleep at all)? This might be a little thing, but if you can't be true to yourself in the small things, do you think it will be easier to stand up for yourself and your feelings when the stakes are high?
Imagine you're waiting for a response on something from your partner and they don't respond quickly (or at all). It might be a small thing, but would you say something about it, or would you just try to deal with it because you don't want to be "needy" or "high-maintenance"?
It's self-abandonment to keep it to yourself—even when it's something as small as a text about what time they're available for dinner. Think about it. If you don't mention this in the beginning, they'll think it's fine—in fact, they won't think about it at all. Meanwhile, the frustration will build and build on your end until you've convinced yourself they must not really care about you at all and you think you have to draw a line in the sand.
Meanwhile your partner has no idea why you're suddenly giving them an ultimatum—"Either show me you care, or I can't do this anymore." You kind of pulled a bait-and-switch on your partner. They didn't know you were upset, and now you're upset they didn't know you were upset even though you never told them.
Do you often find yourself trying to be a different version of yourself when you first meet someone? No, not the best version. The best version of yourself is the person you want to be every day, no matter who's standing beside you. Do you abandon the best version of yourself in favor of an attempt to be the best version of yourself for one particular person?