The verb to leave, like the verb to go, is one of the most common: we use it all day long, without paying much attention to it: "I'm going to work..., to school..., shopping..., we're going on vacation..., I went to the mountains...". It indicates that we are always moving: we could say that our existence is a permanent migration. But, of course, these daily departures are not deadly…
On the other hand, beyond these daily movements, there are more radical departures or migrations to which this expression applies: "to leave is to die a little". It is then about leaving one's place of life, for a more or less serious reason, depending on whether this departure is voluntary or imposed: I can move to go to the place of a new posting: this posting can be more less imposed or chosen; I am going to join my spouse and, in principle, it is rather positive; it can also happen that I am driven from my home by war or conflict, by a climatic event or a disaster: then, I have no choice but to leave to save my life and my loved ones; it's just a run for my life...
Obviously, the cause of departure influences how it is lived. But, anyway, to leave is always to die a little.
Indeed, when we leave, we leave a place to which, in general, we were attached, a place more or less loaded with memories, depending on the length of time we lived there...; we leave behind neighbourhood, friendship, work, and possibly family relationships...; a professional activity; habits built over the years; emotional bonds...; perhaps, material goods that cannot be taken with us... All this leads to a detachment, more or less painful, which makes one think of death.
But when the departure is forced, it is simply a wrenching out from the world that one must leave, more or less quickly and brutally, where one leaves almost everything, taking just the essential with oneself, as far as possible. If we have the time, we then ask ourselves: what are we taking with us? Important papers? Pictures? Clothes? Hardware?... To leave is then, quite simply, to avoid dying; but also, really, to die a little, especially if there is little prospect of return.
Thus, as we are always, in a certain way, on the departure, in a migration, in daily life itself, the proverb is verified, more or less fully, in all the cases of effective departure. But, our lives end, all of them, one day, by a departure with no return that all the departures that we experienced during our lives have anticipated: an ultimate migration, prepared by the migrations of each day which constitute our human condition. Since by nature we are all mortal, we are, all of us, migrants, both in space and in time.