I think a lot of it is in the tone of the critique. Personally, I find really nasty comments to be demotivating. They make me sit back and ponder other uses for my time, other uses that would be more appreciated. As Bill pointed out, we do this for free, and there's really no upside beyond hoping you've had a positive impact on your readers' lives.
Genuine observations and critiques, on the other hand, are just the opposite. Sometimes I'll read them and think one or more of the following:
1. You don't get where I'm going with this. Just hang in there and it will make sense.
2. Wow. You're right. I fucked that up. Usually this involves character consistency.
3. Man, I didn't write that very well at all. The readers didn't understand what I was trying to say.
4. Your comments are interesting, but you're looking at things through your own lens so tightly, you're not seeing what the characters are seeing. This is most common in generational issues.
Regardless, that kind of feedback helps me keep my finger on the pulse of a story. There have been many times when I've drafted several chapters ahead, and I go back and revise them to address issues readers have raised.