I think it might be easier to forget with people you know well. I must say I can't imagine not thanking a relative stranger, and I hope I never forget such...
But with people you know, when they forget to thank you, you can also tell how thankful they really are by the comments, their behavior etc. I totally agree with what you said. Point being, when it's someone I'm close to, that forgets to thank me, that's what I think of. When it's me who forgot to thank them, I'm like you.
A very trusted friend helped me turn 13 pages of pure, unadulterated tripe into my current WIP, which I've shown around to some people. Her critique was honest, blunt and to the point, things like "don't take me for an idiot here," and "I really like this paragraph." Not to mention an assessment of said thirteen pages. If it hadn't been for her . . . I can't even begin to imagine.
A recent beta experience I had recently was with someone who didn't know me so well. I asked her for an honest and blunt critique. She went very slowly at first, seeing how much I could take. It was a short story that had received very positive feedback from some sources. She basically ripped it to shreds, but in the best possible way. And I thanked both of them profusely.
Different readers will react to different things, some will like your style and some want. But instead of snapping at them, take that on board. Because, no matter how good your story is, there's always someone who won't take to it. So, even if you don't see eye-to-eye with them, it's worth taking that opinion on board. I remember one person in a crit group commenting on a story, "this takes place in another country, and I don't live there, so I'm not interested in the story."
I guess finding a good beta really is like playing the lottery.