But why not vote for a loser? Is it just some irrational pride/feeling thing? One vote is never going to determine the outcome anyway, yet a spoiler vote is still a signal to A and B about how competition is stealing their votes and how they could win you back.

That other reason you mentioned is ridiculous too. Since parties A and B always win, alternating each one or two cycles, it's not the end of the world if your hated one wins this time - if they don't, they'll just win next time anyway.