The reason why heaven and earth can be enduring is that they do not give themselves life.
How to think about “giving yourself life”? It’s those ways in which I try to add to my life, to qualify the existence I’m given, as better, fuller or richer (materially – eye-catching clothes; spiritually – career success, achievement). To “give yourself life” means to make your life an object with a meaning, to objectify yourself to others or to yourself. It is to identify with the life you create or imagine for yourself as your “self”.
Therefore the sage puts his person last and it comes first,
Treats it as extraneous to himself and it is preserved.
“Extraneous to himself”? If we consider “my person” to be “persona” (mask) – the life I have created or imaged for myself, a “self” – then the sage correctly considers this person, this objectification of his existence as self, this life he has created for himself, as something extraneous to himself. He disassociates his existence from him being celebrated or castigated: he lives well by centering his thoughts and actions on the most fundamental level of existence, as the exitsence of a being among heaven and earth.
Is it not because he is without thought of self that he is able to accomplish his private ends?
Eschewing selfishness – that is, not relating to himself as to an object he serves – the sage can accomplish his private (that is, “inner”?) ends, like clarity of thought and sensation, and longevity.