A 51st-minute goal from Josip Brekalo proved to be the match-winner as Stuttgart overcame Dortmund 2-1 at Mercedes-Benz Arena Friday.
Earlier, Marc Bartra and goalkeeper Roman Burki had gifted Chadrac Akolo the hosts' opener before Maximilian Philipp equalised after Andre Schurrle's penalty was saved.
Bosz, whose team are now winless in five league matches, was left upset by the result and the way his side conceded.
"The defeat really hurts. We came here to win, so we're very disappointed," he said.
"When you see the goals we conceded, it borders on the ridiculous.
"It hurts because we actually put in a relatively good performance in the first half. The team performed well after conceding the early goal, only the final ball was lacking.
"The second half wasn't as good. We need to keep going, we won't give up."