Our sense of smell is one of the strongest senses we humans possess. Have you ever noticed how a certain smell can trigger a flood of memories or instantly change your mood? For instance, the scent of freshly peeled oranges always reminds me of happy childhood Christmases, and it only takes one sniff of lavender in my bedroom to make me feel instantly calm and sleepy. Well... it turns out the reason for this is that there’s a direct link in our brains between memory and smell.
To be scientific for a moment, our olfactory system (which is responsible for our sense of smell) has direct access to the hippocampus (which is responsible for associative learning) and also the amygdala (which is responsible for processing emotion). This means that when you smell a particular scent, your brain automatically tries to link it to an event, a person, a thing or even a moment you've previously experienced, and this can alter your mood instantly.
Because of this we can actually use a scent to create happy long-lasting memories, for instance deliberately creating a 'happy smell' memory by choosing a particular scented candle to burn at a wedding or other special occasion, that will forever bring back a flood of happy associations for years to come with a simple sniff!