I also ask myself that same thing...I so wish I had an answer... for me it is always a "smell"... a certain "smell"..
I say to myself.. it is such a little thing... but then another part of me says "but it is an important 'little thing' " reminding me of where I have come from.. to where I am now.. and then.. I stop and say to myself.. I am glad I am here - in the now.. not back in the "there"..