Friend who is the same in happiness and sorrow
In indo-european folklore the common expression "the same in happiness and sorrow" usually means one who stays a loyal friend not only in happy times but also in times of trouble, when having all responsibilities of a true friend could be taxing. This corresponds with #3 on the list, "in misfortune he does not forsake one".

By extension, "the same in happiness and sorrow" means one who is not seeking a benefit for oneself from the friendship, but sincerely gives oneself away to the friend. This is where the other three come from:


  (i) he reveals his secrets,
  (ii) he conceals one's own secrets,
  ...
  (iv) his life even he sacrifices for one's sake.


"Warm-hearted" means a friend whose relationship of trust and commitment comes from the heart, is unforced and natural. Such friend would not have to make an effort to "sacrifice his life for one's sake", it would come naturally.

Altogether, the above is not to be understood literally, but is meant to portray the attitude of uncalculated selfless fondness, almost love, as the perfection of friendship.