Why was persephone not permitted to leave hades permanently?

Because he wanted her to be his wife forever.

Explanation:

it is natural to stand at the beginning of a research project and feel overwhelmed by the amount of published research that exists in databases, literature reviews, and reference pages. at the same time, each new research project brings the hope of discovering something new. overwhelming though a project may be, starting at the foothills of a new thread of research is a great privilege, and is best approached as an opportunity to learn rather than a drudgery. as a researcher/writer, you have the chance to dive more deeply into less frequently encountered pools of knowledge.

depending on the topic or scope of your research, it is also natural to spend many days and weeks - and in some cases months and years - searching. no matter how great or small the scope of research is, the serious researcher needs to reserve adequate time to perform a thorough survey of published articles. for an undergraduate course project, finding five or six sources might seem like plenty of material to review, but graduate-level writing projects typically involve up to 20 sources minimum.

note that the main point here is not to say that it is only the number of research articles matters most, but rather that having a broad spectrum of papers to choose from you choose your topic for at least the following two reasons: 1) a larger pool of sources provides you with a broader perspective of the topics within your scope of research and 2) along the way you will find many topics within your field that you do not want to write about! so, one particularly effective way of viewing research is not finding the absolute minimum sources to "get by", but rather to find a variety of sources that you can an artist uses negative space to "carve" shapes out of a dark guide you toward topics that are more directly relevant to your topic.

the good news is that as you research you may find that some of your sources that were published in the same decade or so will cite and reference each other.

one of the joys and privileges of research is being able to follow your curiosity; if you are truly curious about your topic, and authentically driven to find out as much as you can, then even the articles you don't find interesting will be useful for a future project, and no energy will be wasted.