I got asked this question by a friend who is trying very hard to quit smoking.
He wanted to know why humans have this unique self destructive tendency. Since the conversation was over whatsapp, I didn't reply in length.
This is the answer I came up with when I found some free time.
All life, Humans included, will try it's best to acquire and conserve energy.
That is the basis of life.
We have evolved to eat as much as possible when food is available, and laze around as much as possible to conserve all the calories that we gorged on earlier.
The problem is, with humans, we've gone from a time of limited energy availability to a perpetual energy excess. Excess food and labor saving devices. And since we're still hard wired to manage energy in this way, we'll get obese if we don't watch out.
The further we move away from our evolutionary background the more difficult it will be to stay healthy.
The more unhealthy one becomes, the more stingy the body will get with spending energy and more interested it will be in accumulating energy.
Ever wondered why you feel more lazy and hungry as you continue to put on weight? That's probably millions of years of evolution trying very hard to save you in a situation it has had very little experience dealing with.
The other side to this is, the foods we eat these days are more strongly flavored and more energy dense than what was available a few thousand years ago.
Taste and flavor is what humans and other animals use to decide which did source is more nutrient dense and energy dense.
Sweet foods are energy dense.
Salt(rock and sea salt) is trace element dense and has sodium in it that was rare in paleolithic times.
Sour taste in natural settings usually indicated high vitamin content, probiotics.
Umami indicated high quality protein.
A combination of these flavors was even rarer in times where the culinary art hadn't been developed.
Then, modern science happened. That technology got added to the already well developed science of cooking.
We learnt to manipulate flavour. We learnt to manipulate texture, consistency.
Now we have taste scientists working in well equipped labs running on budgets of small cities, trying to find ways of getting you to eat more of their products.
Do you really think you stand a chance?
Combine that with a high stress lifestyle that spikes your stress hormones which in turn, spikes your cravings.
Add high decibel, in your face advertising by these companies, and the battle is already lost before you start.
That explains why humans like eating stuff that they know is "bad" for them.
If your next question is," How do I fight it?"
I'm sorry, the answer is long and complicated and will need a book to explain. Plus, the approach will need to be tailored according to individual requirements and lifestyles.