Okay, I know this is a crazy question, but go easy on me (I'll restate it below in more precise terms): Why would merpeople (mermen and mermaids) allow someone to ride them?
Such a ludicrous question demands some background information, so here goes:
The setting is a coastal kingdom (the Mainland) and the islands it holds power over (the Outlands). The sea is very much a part of their lives, and so merpeople by extension are a part of everyone's lives.
The humans want to ride merpeople because they swim better than humans (they're faster/more agile), and for much longer than humans. Yes, they could use boats, but riding a merperson is cooler/more fun/less lonely/why not? It could be a status symbol, maybe?
The merpeople think it's ridiculous that people want to ride them, but some of them are okay with letting a friend ride them (think a piggyback ride; that's not a big deal now is it?) or getting paid to carry someone. Others are curious as to why humans suddenly want to do this and want to investigate and/or find out, while others are curious and want to try it out just for the sheer novelty.
TLDR: Restated, what could possibly explain merperson riding becoming more than a fad, a nation-wide (if not world-wide) phenomenon?
(Ex: Pokemon. Having two creatures fight each other has been a mainstay of human entertainment for many years, as evidenced by Roman gladiators, cockfights, and dogfights; but only in Pokemon and its various ripoffs does animal v. animal combat become an absolutely massive, inescapable part of society. That's kind of what I'm planning here if that's possible).