There are a few sections in the Help Centre that cover off what is / isn't appropriate.
One specifically about this issue is found in the How to ask a good question section, which contains:
Make it relevant to others
We like to help as many people at a time as we can. Make it clear how your question is relevant to more people than just you, and more of us will be interested in your question and willing to look into it.
There is also quite a bit of information about what sort of subjective questions are appropriate, such as in the What Not To Ask section.
Across many of the Stack Exchange sites, there is a bit of a problem with people using the sites as their own free consultancy service rather than taking the site(s) as they are intended - as a Q&A repository.
In an ideal world when someone has encountered a problem in their professional life (and sometimes personal life) that they know there's a good solution to but can't figure it out themselves they would type that question into a search engine and a Stack Exchange site would display in the results with that very question. However, if questions are more like "Which of my logo designs is the nicest" then that's never going to be of use to you, because you'll never be using that particular logo. Nobody is ever going to search Google for such a question, nobody would open that question on the site to view it - it'll only be used by the person who asked it.
That adds noise to the overall set of questions and reduces the usefulness of the site.
You can look at the questions across the site. Although you may not have come across 99% of the problems yourself yet, in general they should be questions that make you think "Ah, if I ever come across this situation then I will be able to use this question as a good point of reference for coming to a solution".
Such subjective questions an often be made more general with some rewording though, which is generally what we encourage. But not all questions are salvageable.