I was curious myself about this phenomenon and looked it up.
When you first light up your tobacco you can smell it. That glorious scent of fresh tobacco being loaded into the pipe - the sulphur of the stricken match head burning off when you bring it on top of your tobacco. Then, you start to draw in on the pipe and the scent and flavour starts to become heavenly.
Your nose becomes accustomed to the smoke and scent. This is called Olfactory Adaptation – or ‘nose fatigue’ – and is all to do with how our body senses danger.
Once we become used to the smell of the tobacco, our body decides it isn’t something threatening, and it blocks the smell; making our nose available to new scents and anything potentially harmful. This is why we can oftentimes smell someone else’s cigar or pipe smoke side-stream and it’s awesome. We feel that ‘our’ experience (our own pipe and tobacco) are lame-ish by comparison.
But on a good note, our sense of smell is the biggest pull for nostalgia. It can bring us back to different times in our lives and conjure up strong memories. Who doesn’t remember a fond friend or beloved relative who smoked a pipe or cigar?