The fixatives. It can be the strongest scent, and it will still fade quickly without a good fixative. They are usually waxy or greasy substances that hold the scent on the skin. On a microscopic level, it's rather clever how it works. The oily fixative floats on the surface of your skin, on like oil on water. The scent and other ingredients float within the wax, and since the scent and alcohol base are less dense than wax, the scent floats up through the wax and is slowly released, being carried away by the heat currents of your body. Without the heavy oil/wax to move through, the scent will float away quickly from your skin.
Now I will scare you, some of the best scents may not be cruelty free. Ambergris, basically waxy whale barf, is an expensive but wonderful fix for perfume. It can be harvested naturally, but harvesters sometimes injure whales or other sea life to get it, because it's rare enough to be extremely valuable. Other perfumes may be made with civet cat musk, and generally the cats have to die to get their musk glands. Most companies, including Chanel, now use synthetic or cruelty free fixatives, but some high end scents may still use cat musk or ambergris that was not gathered nonharmfully.
Hate to say it, but the vintage Chanel perfumes that I'm sure have civet musk in them last twice as long as the newer ones, and ambergris perfumes will last until you wash them off. Do what I did, raid your grandmother's perfume stash for vintage scents. That way you aren't buying perfumes that may harm animals, but you still get the old school No. 5 that smells great all day.