How do you ‘marry’ this phrase with the ‘there’s no such thing as a free lunch’?
I firmly believe there is no such thing as a free lunch and also that it ‘cuts both ways’.

One thing that also comes into mind is that scarcity raises the value. Would we cherrish something if it was neverending?

So, goodbyes give relationships more value. When we are aware of them (the permanent ones) we favor and enjoy the time spent together.