Why do I dislike a job I once enjoyed?

People's interests change, and that is completely normal. You are allowed to change your mind. So if you originally liked a job, maybe you don't like it now because your environment or personality changed.

It could be due to monotony or stress. Anything done with positive and clear mind will be enjoyed better. So try to be positive and stress free to enjoy the job again.

I would first try to look at what aspects do you believe is causing you to dislike your work. Once you are able to identify them, try to see how they can be adjusted.

Because people change. It's against human nature to stay with one thing forever and ever. You're not the same person you were when you started your job and it's only natural that with time, your opinions start to change. Maybe you become bored. It's all understandable and perfectly normal.

Maybe things have changed - for example your schedule, the workload, the people you worked with or the environment of your work place. Some people get bored of their job and seek to find something new. Make a fresh start and find something else you may enjoy.

it can be vacause now you are a upgraded version of yourself, and as the way you think changes your world changes

Two possible reasons; stagnation and complacence. Stagnation occurs when excitement wears off and you become dissatisfied with your general place in life. Complacency occurs when you start to get about the things you are grateful for and look for other things - it's human nature and very normal. In this case, you may need to step back and reassess what you love about your life, what's missing, and where you can go from here.

Your personal preferences can change over time. People are always changing and growing and that can lead to enjoying new things and not enjoying something you used to.

Along with new acquainted professional skills our preference and enjoyment change too. It's only natural. You can evaluate why you stopped enjoying the job to figure out if you want to regain the excitement or if you should consider looking for new challenges workwise.

It is totally normal to not enjoy things that you might have once loved. It might be because your perspective changed. Maybe your job includes more roles and/or responsibilities that it once did not include. Maybe the people that you work with have changed. There are literally so many different possibilities! It could also be something as simple as you. You could be growing as a person and might find your job not fulfilling your ability and/or drive. Something that might have taken you lots of time and energy, might be very easy for you given your experience in your job. The important thing is for you to find ways to make it exciting again, through new experiences, or move on.