Life-long friendship means that the friendship between two people is very good, which can be the kind where each person entrusts his life to the other. It is a kind of trust in each other, with complete trust and dependence on each other and absolute trust.
This kind of friendship will go through many tests of life and death. It is a kind of trust that the other party always sticks to it when facing desperate situations and great risks, because in desperate situations, they can always be willing to take risks.
In this case, the most common thing is friendship, especially when we went to the battlefield together and gave our backs to each other, but the other side didn't want to disappoint themselves. Or the police, etc., they have all experienced taking bullets for each other, and only after such experience can they have a life-long friendship.