We often are ungrateful and if we look closely, we are blessed with so many things for free, be it free oxygen, water when we are thirsty, opportunities to work and earn money and the possibility to buy good foods and experience this universe. I guess it's human nature to forget importance of stuffs which are available without asking

That is life for you, people don't always appreciate the little they have. Human have that nature of not being grateful, but I pray we should learn to get the attitude to be grateful with everything we have around us.
that is correct - it is human nature that when people do good to you - you are happy but when they are unable to help you or when there is something which you dont like - you never think of the good thing - you hold the grudge and treat the person on that condition.
But staying calm during trouble time is very important. so that you don't repent later.