Gratitude! A highly over-rated sentiment?

"Be grateful." "You ingrate!" "I have done sooo much for you..." And unspoken: "I have helped you when you needed it and now look how you treat me! I am not your friend anymore!"
How often have we all been at the receiving end of such sentiments? Or have thought these thoughts ourselves. But why do we all do this?
On a person to person basis we have all been helped out by friends and family. Or helped others out - people whom we are fond off or close to or to whom we owe a good turn to. But why do we all expect gratitude in return? Why can't human beings (or nations) help each other out without expecting the other party to be forever grateful?
One old friend helps out another old friend at a critical moment and then gets upset because he or she doesn't always jump to her/his bidding. "After all", the reasoning goes,"I helped out in a big way so why won't he/she jump to my bidding?"
Or one BIG country helps out a not-so-big developing country with vital food grains and then is completed shocked when the debtor refuses to kowtow in all matters of political philosophy.
And then you have the parents! Which parent (even the most tolerant, nicest, understanding, stay out of the child's hair kind) hasn't at some moment or the other thought: How can my child treat me like this? After all I have done, given, sacrificed, etc etc? (Is it more honest to expect gratitude and expect something in return - as in "I put you through IIT now marry the girl/boy I choose for you or look after me for the rest of my life, screw your wife's/husband's feelings" - or shut up and suffer feelings of hurt and rejection?
Why is it that anonymous donors can give big time to charity without expecting 24/7 gestures of gratitude ("No, no", says the donor, "it's nothing really!It's the least I can do!") but when it comes to personal gestures of help and handouts, then a lifetime of gratitude is the least one can expect in return.
Why can't good deeds remain just that, good deeds, with no expectations of returns on investment?
Is it because this flow of gratitude makes us feel better about ourselves? Helps to bolster our feelings of self-esteem? Makes us feel like we matter to some people at least? And gives us a raison de etre for living?
Why can't we just do good things for each other without expecting to be told that we are the greatest? That we really are the next best thing to sliced bread? That we make a difference in this uncaring and careless world?