Most Helpful Guy
You have to realize that for almost all of human history (and even now in too many places) starvation was the greatest danger we faced. Unhealthy food does one thing well - it makes us fat, which used to be the MOST IMPORTANT THING IN THE WORLD. That's why we love it so much.
Most Helpful Girl
The fast food industry and junk food industry literally spends billions, yes billions, per year to perfect our evolutionary taste for fat, sugar, and salt. Naturally we all need and crave these things and they have helped with our survival, but the food industry has made it their literal business to ratchet up these things to astronomical levels where it has built up a literal addiction in some people for the food. For example, if you are a heavy coke drinker, and you consume 2-3 bottles a day, the moment you quit cold turkey, it is like you've quit taking drugs. You experience anger, headaches, you continue to crave it, etc. This is why it takes about 3 weeks to start really ridding your body of the need for these... well... substances and addictions.
Now if you start eating healthy food purposefully and regularly, you will learn to like it over time. It can be very tasty and healthy for you at the same time, but that doesn't mean you will completely stop craving the fats, sugars, and salts found in addictive fast foods, but perhaps you learn to crave them a bit less.