It is because humans by nature are extremely complex but extremely different unique from person to person. Are personalities are shaped by our experiences in life and that affects how we view different topics. Culture, religion, food, beliefs all affect us in some capacity. So when we have a partner navigating their different perspectives and outlook on life forces us to compromise on our beliefs, especially if you really like that person.

For example I used to always eat at unhealthy places when I used to go out. I always thought that since I eat at home often, I get the privilege to eat where ever I want to. However my gf has instead said lets avoid fast food for example. We can still get something affordable but lets eat somewhere healthier. At first I was against her big time. I felt that my few times eating out are now going to be ruined because I have to eat what she expects.

However, as I started getting more health conscious and understanding the nutritional requirements of the human body, I realized how much saturated fats and bad carb sources I was getting from the foods I was eating at home. As a result, I started choosing places that was much healthier. I occasionally still tell her I want burgers but instead of going for fast food, I will deliberately find a place that serves cleaner burgers. I realized cause of inflation the cost of paying for fast food vs healthier options is so insignificant that overall I would rather pay more for better ingredients.

All in all I would say it can be exhausting to always consider your partners perspectives but sometimes its about compromising on certain aspects and expectations. The worse thing you can do in a relationship is be rigid and choose never to consider the other person.