Whoever makes the incense like that in order to smell it shall be cut off from his people. (Exodus 30:38)
After describing the composition of the incense, the Torah warns that it must not be copied in order to derive personal pleasure from its smell. The fact that the mere smelling of the incense was prohibited implies that smell is an equally important way of deriving physical pleasure from the world.
The Shulchan Aruch (Code of Jewish Law 216:1) rules that just as it is necessary to recite a blessing before enjoying food or drink, a blessing which be recited before smelling scented trees, plants or spices (see the Chief Rabbi’s green siddur page 780).
These blessings are restricted to an object whose purpose is to give off a pleasant smell (ibid. 217:2). Therefore, if one entered a kitchen while food was being cooked or walked past a bakery, no blessing would be recited over the smell of the food as it is not being prepared in order to produce a smell, but rather to be eaten. Similarly, cut flowers are usually bought for their aesthetic beauty, not their smell and no blessing should be recited for their scent.
No blessing is made over man-made scents or over deodorants and air fresheners which are designed to remove or prevent bad odours (Mishnah Berurah 217:10).
Although there is also a blessing after enjoying food or drink, there is no blessing after enjoying a smell as its effect disappears as soon as the source is removed.
The Gemara explains that unlike the other senses, smell has the unique capacity to benefit our souls rather than our bodies (Brachot 43b). The brain interprets signals from our nose via the olfactory bulb. Since this is part of the brain’s limbic system which is related (among other things) to long-term memory, smell can elicit deeply emotional memories and instantaneously transport us back to our past.
This is why we use spices during the havdalah (separation) ceremony after Shabbat, to stimulate our souls as we make the transition from the spirituality of Shabbat back into the mundane week (see Chief Rabbi’s green siddur page 608).