Taking someone for granted is the act of not cherishing the deeds another person does for you by not reciprocating them or being ungrateful for them. It also means not being satisfied with what another person offers you.
1 Additional Answer
Taking someone for granted means expecting someone or something to always be available to serve in some way, without recognition. It could as well be looked at as valuing something or someone too lightly. Taking others for granted sometimes leads to misunderstandings as those being taken for granted may feel unappreciated.