Asking people to vote for your work means you think your book would be a worthy winner, and that can look rather big-headed. Or even worse, it suggests you don't care if your book is the best: you want to win anyway. Both of those can rub people up the wrong way.
For example, just before Christmas I saw a guy on Facebook saying that "anyone who hasn't yet read my ---- can assuage their guilt by voting for it in the ---- awards". Now, that is exactly the worst of it: you haven't read my book, but vote for it anyway. Ptui!
Awards are nice, and I'll admit that, now TQF is eligible again for the British Fantasy Awards, I'd be very excited to pick up another nomination, even if I don't think we quite deserve it yet. But they're not worth being silly about. Be cool. Encourage people to engage with the awards process properly, to read as many nominees as they can, not just vote for you because they're your pals.