to share happiness and grief....

तदेवास्य परं मित्रं यत्र संक्रामति द्वयम्।
दृष्टे सुखं च दुःखं च प्रतिच्छायेव दर्पणे॥
सुभाषितशतके॥
tadevāsya paraṁ mitraṁ yatra saṁkrāmati dvayam|
dṛṣṭe sukhaṁ ca duḥkhaṁ ca praticchāyeva darpaṇe||
subhāṣitaśatake||

the test of genuine friendship is defined in this Subhashitam.. taken from a collection Subhashita Shatakam.
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meaning
This indeed is the ultimate test in friendship..
In genuine friendship when one sees happiness in a friend , he too becomes very happy.. And when he sees grief in the friend, he is filled with grief too..
The combination of the two is the real test.
The happiness and grief are reflected by him just like an image is reflected from a good mirror.
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Spontaneity is the foundation for genuine friendship. The responses should come without any editing or manipulation..
A true friend would just reflect the feelings of his own pal just like a mirror..

Today I had a discussion about this issue with a learned friend..
There the depth of friendship was discussed at another more profound plane
My remark there on the mirror analogy was

"Not only that ..a mirror can reflect only to the extent what the source has.
The light can reveal only what is available.
But a friendship can produce and give what we do not have.. and that too without asking."

Of course fast friendship can go much deeper..
But the idea conveyed by the subhashitam is very valid.. only that we can build up further on that..