If a person doesn't like to spend long hours studying a certain subject such as math, they might think they're bad at it. In this case, it's the lack of studying that decreases a person's mathematical abilities.
Failing an important exam in school, for example, can motivate a person to hate math and make them believe they're bad at it.
One reason why some people hate math is the way they were taught the subject in schools. If the teacher was mean or unhelpful, for example, it could have resulted in students hating math and not being good at it.
Like any other subject, math needs continuous practice in order to become familiar with all the rules and numbers. If a person doesn't practice enough, they will never be good at it. See Why some people are better at math
Studying math needs a trial and error mentality. Giving up after attempting to solve a mathematical problem will only increase your hatred of math.
According to Wikipedia, math anxiety is the feeling of fear and nervousness when facing a mathematical problem. Many students suffer from math anxiety.
Some people may have a serious fear of math, numbers and calculations, which is called arithmophobia.
People who don't like thinking about logic puzzles may develop a hatred of math.
A person could be bad at math because they are controlled by their fear of failure. In other words, they have become accustomed to hating math that they don't want to try to become good at it.
According to a Quora thread, an adult could be bad at math because they lack the foundation, which is usually built during childhood.
When a person finds a certain subject or field boring, they stop trying to understand it, and consequently become bad at it.
A person could be unable to carry out mathematical problems because they fear they will be embarrassed in front of people if they don't answer it correctly.
Some people may be unable to solve math problems during exams due to the time pressure they're subjected to.