There's three different levels of boundaries that I run into in BDSM. And hell, regular sex too.
1) What I actually enjoy--more than this and I won't be having fun.
2) What my mind can tolerate--more than this and I'll get genuinely frightened or angry.
3) What my body can tolerate--more than this and I'll actually be injured.
I tend to play like enough of a fluffy little bunny that 3 is almost never a risk, but sometimes it's hard to make the distinction between 1 and 2. There are times that I'm willing--even find it somehow important--to go to level 2, but those times are few and serious. When I'm just having funsies with my budsies, I really want to stay within boundary 1. And that can be hard to communicate. It's a downer, and sometimes considered uncouth, to "yellow" or "hang on, honey," not because of any danger but just because you're not getting off. But it's a much bigger downer not to.
Hanging in there because you're able to put up with something, while letting your partner think that you're just lost in ecstasy, sometimes feels like the easy or polite thing to do. It's also a great way to guarantee that you get the same thing next time, and that your partner never really knows what you like, and that sex feels a little bit like a chore. As so often is the case, being polite is nice, but being so polite you don't communicate isn't doing either of you any favors.