You cannot rescue a drowning person if you get dragged down in the process.
Simply put, a person who is struggling to survive will not, can not, simultaneously also consider your well-being. In his or her thrashing to stay afloat, he or she may very well pull you under as well, causing you both to drown.
This is not a conscious or logical choice on the part of the drowning person. It benefits neither of you, but biological necessity has no foresight.
In order to successfully and safely save a drowning person, you must first be in a position of safety and security yourself. This is not selfishness. Both yours and his or her survival depends on it.
When someone cries out in need of help, first consider whether assisting them will pull you both under.
Holistic Budo: As it is in budo, so too it is in life. As it is in life, so too it is in budo.