Actually, there are lots of appointments that people want for different reasons. The ambassadors he's appointed in '17 are a mix of business cronies, old party warhorses, donors, and a couple of professional foreign service people. But while some consider the western democracies as the plum posts, others want other countries. The appointees to Costa Rica, China, Croatia, and the Czech Republic, for instance (just going by the "political" postings under the letter C in '17).
I don't know the backgrounds of all those people but one assumes the purty lady who got Costa Rica must've asked for it. Or Scott Brown certainly wouldn't have been pegged for NZ if he didn't want it. Maybe he skis? Maybe he sails? Has a thing for Maori history? And I'm sure K.T. McFarland must've asked for Singapore.
But, yes, there are a whole lot of career foreign service people, like in Thailand. No one asked for Bangkok and there's a dicey relationship, so they gave it to a likely suspect in the professional ranks. In years when we like Thailand (on paper), there are people asking for the post. (Largely ceremonial, most action is in the Commerce Dept attaches and Immigration people.)