It doesn't matter that the person is not here to stay. She is here, right now, in front of me, and I can choose between wasting this moment with her or enjoying it. And I pick "enjoying the moment", of course.
And yet in the last couple of years I started to think about the goodbye part a lot more. In the past I sometimes tried to forget that a goodbye was going to happen and to focus on the present moment instead, as already said.
And is it important to me or not at all?