many times i have this kind of experience: like someone does something sweet for you and you feel really really paiseh because you feel like you don't derseve a spit of what was done.

familiar?

hahahaha.....if i may so say that this problem is some sort of a indecipherable H2math question, then perhaps i've found my personal answer.(whether the "teachers" marking this question thinks the same is a different matter entirely)

anyway, what i thought was that if people want to help you or make life better for you, it doesn't only mean that you're the only one benefitting. the person doing that very act, has also "benefits" by seeing his/her gestures appreciated. it doesn't need to be an group-hug-thanks-alot girly thing..it doesn't need to be a verbal-thank-you (although it'll be very welcomed) but things like a simple shy smile, a simple blush also contributes to the gratitude you show. in doing so, you've already given back to the person who gave it to you in the first place.

感恩：是要去感受，要用心来回报别人的用心良苦。

only those who do not show gratitude and show gratefulness what ever that has been given, that do not "deserve" it. sometimes - pardon my subjectivity - i just want to slap these kinda people up down left right center....

in your terms: newtons law states that for every action, there is an equal and opposite reaction.

if you give,(action) what you get is gratitude (reaction).

if you receive (action), you benefit from the act itself but you also give back the smile in the form fo yours (reaction).

i guess whatever you study can be applied to daily life no?

ask not for the answer, instead, ask the right questions......

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this world does not consist of just ourselves. overtime, people grow attached to each other and their feelings, regardless of how we feel, becomes our "responsibility".

it is a "responsibility" that we think for them as well, while considering our own feelings before we do or say anything.

sometimes there may be slips and falls, but ultimately, people look at the status quo and friends who know you long enough understand that sometimes we all need our space and time to sort things out.