English dictionaries define the noun "man" as:

- a male

AND

2. a member of the species Homo sapiens or all the members of this species collectively, without regard to sex: e.g., prehistoric man.

3. the human individual as representing the species, without reference to sex; the human race; humankind: e.g., Man hopes for peace, but prepares for war.

4. a human being; person: e.g., to give a man a chance; When the audience smelled the smoke, it was every man for himself.

In defintions 2, 3, and 4 the word man is used to include all people, males and females.

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To see the defintions and what Andi is talking about, read the 'Usage Note' at this link:

http://dictionary.reference.com/browse/man?s=t

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"The man is ungrateful creature" should be written as:

-  "Man (all the people) is AN ungrateful creature", OR

-  "Humans are ungrateful creatures", OR

-  "Men and women are ungrateful creatures."

If we write "the man" it means one male person.

I think your example is about good persons, at least, they wonder: "why...?" The most of  people always know an answer...:

"-Because of YOU!!!!")))))

QUAN, if humans were grateful there would not be wars in the world.

Leyla, are you talking about men, in particular' Or people in general?

In English these days we don't use 'man' or 'men' to refer to all people, especially in the developed English speaking nations where gender studies are important. 

VIKTOR, probably you are right:)

RICHARD, thanks a lot. I will read. Again your useful lesson :)