My oppinion is that it is always related to research quality. High quality papers get more citations, since they are useful for other researchers. Self citation can also help a little bit, but I wouldn't recommend to anyone to self-citate just to increase h-index, but to do it because it is actually necessary.
You might want to take a look a this work: https://www.academia.edu/934257/How_to_increase_your_papers_citations_and_h_index_in_5_simple_steps