Mostly, it's the author. If I enjoy Book 1 of a series, I will read Book 2 next, and so on. But if the book is standalone (or end of a series), I'll just look for another book by the same author.

I have become wary of starting a very long series (anything more than 5 books). Very few authors can keep up the pace and quality over a long series, and there's always the danger that they'll never finish. I like knowing there'll be some conclusion eventually!