I think some people are raised in a culture of criticism and as a result think that being a critic of all-things/people-imperfect (in their mind) amounts to trying to help someone else become "perfect". In other words, many people equate criticizing with teaching.
So there can be that. Then, mixed in with attempting to be "teacher of others" can be a tendency for some people to prefer to judge others (often unfairly) because they get to feel better by seeing/imagining the worst of someone else and/or by not seeing the best of that someone else.
Having said that all that, though, on the side of the people who may not seem to see the good that someone does (or does for them), they often do and may they really appreciate it. One problem can be that even if they do there may be other things going on for them that makes whatever good deeds/help someone else offers/provides not enough for them (not because they don't appreciate it or because they think they should even be in the position of having to be helped by someone else; but because they are individuals in this life, on their own terms, and aren't going to be able to be happy unless/until they can live their life that way.
People can appreciate and/or see "up to their eyeballs", but there's only so much saying "thank you" one may be able/willing to do (because it can eventually start sounding hollow - or else because the other person is expecting more appreciation for his efforts/contributions than is reasonable.
On top of all that, there can be people who try to do something for someone else but do the wrong things (because, maybe, they're guilty of the first things I mentioned and therefore, in all their good intentions, keep doing the wrong things "for" the other person. And, there are also times when someone often does the right things for someone else, but also the wrong things. So, people can appreciate and "see" the good things while also be unable to overlook some of the wrong things.
They may do the wrong things because they have leanings toward "selective narcissism" (which makes them see/need to see the other person as "less" but even as "just like them - only flawed/imperfect when/if the "less" person shows signs of being an individual (and expecting to be treated that way). They may not be "real" narcissists, but it's a narcissistic trait to view oneself as the victim (and in such an instance, the victim of the "unappreciative" other who "doesn't see all the good things".)