Notional Loss

I've observed this quality in many people. Perhaps this is how mankind thinks. When you get something, or you are expecting something you don't deserve or have not expected, you become joyous, but when you lose it or don't get it, it hurts. For example, give a gift to your sibling/friend everyday. Don't explain to them why you are doing it. Do this for 15-16 days. Now stop giving gift. Your sibling would ask you why you didn't gift him/her and would expect the gift from you. Let's take another example. If your dream girl calls you to go for a date, you are elated but again when she calls to cancel the date... why do you become disheartened? Remember Uday Chopra from Dhoom 1 and 2.
Why don't we think prudently in such situations? If the things are not meant for us,why do we sadden because of the notional loss? We become emotionally attached to the notional gain we make and start claiming right over the notional profit and then notional loss becomes emotional loss.
What the hell I am writing! That's it for now. Hope I've conveyed what I wanted to.