What is the difference between why do you stick to it? and why you are so stuck on it? ?Feel free to just provide example sentences.

To "stick to it" generally means "to persist in doing it". It has a neutral or positive connotation. To be "stuck on it" generally means "to refuse to let go of a topic, issue, or idea". It has a more negative connotation.

When someone says "Why do you stick to it?" I imagine they are asking someone why they persist in a certain habit, hobby, task, career, field of study, etc.. The words alone don't indicate whether the speaker is encouraging or discouraging the listener. Only context and tone can indicate that. On the other hand, just the phrase "Stick to it." is often said to encourage the listener to continue doing what they are doing.

When someone says "Why are you stuck on it?" I imagine they are asking someone why they keep bringing up a certain complaint. For example, if someone kept telling me "You should really get a bigger apartment.", I might reply "Why are you so stuck on it?" to send the message that I want them to stop telling me about it.

On the other hand, you could use it in a similar way to "Why do you stick to it?", to ask about a hobby or pursuit. For example, if I had a friend in school who always told me information about Biology, I might ask them "Why are you so stuck on Biology? Don't you care about other sciences?" to inquire about what they find interesting about that topic in particular. However, be aware that this phrasing could come off as rude to someone other than a friend.