So even if you don't have to, per se, renounce your original citizenship, you do have to get rid of your passport? I understand sacrifices are made to become a military officer, but giving back my passport would mean I could no longer travel to my home country for the duration of my military service, wouldn't it?
My situation is potentially worse. As a Brazilian/American citizen, AFAIK, native Brazilian citizens are not allowed to renounce their citizenships (only naturalized ones can). Therefore, I can't apply for a passport after giving mine back to get a TS (for obvious reasons) and I can't go to the Brazilian embassy and get a VISA for my American passport because they recognize me as a Brazilian.
Brickhistory, I don't think your question has been answered yet. There is a reluctance because people develop an attachment to the place they have been born and grown up in. Having family in the original country and being part of the culture in your formative years only enforces that attachment. That being said, I do love the US and would defend it like any other American.