It isn't their joy that they keep selling star players, Dortmund doesn't have any other reliable sources of income than trading with players.
Borussia Dortmund manager Edin Terzi is in ideal positions to achieve his objectives, but the board keeps releasing key players, jeopardizing the club's chances of winning the Bundesliga. If one can recurred the club's past and present deals, they sold out talented players who, if present in the club at the time, would most likely have a different story. Borrusia Dortmund are more concerned with trophies; they need to start winning titles because it's becoming clear that they produce their funds by transferring their talented and promising players to outstanding teams, which is a huge loss on their part.
When Dortmund failed to win the Bundesliga last season, I don't think it was entirely because they let go of their stars because at that time Jude Bellingham was still with them, it's just that he had an injury that prevented him from playing, so that was one of the reasons for the defeat, and it is also undeniable that the player Bayern Munich are stronger mentally when under pressure, the experience of winning back to back with a couple same squad gives them that advantage.
Selling stars looks like Dortmund will continue to do so, maybe for them, the money made from selling their stars is more lucrative than the money made if they become champions, and their business has been like that for a long time, even when they didn't want to sell their stars, the players who being targeted by the club is very likely to leave if there is a more tempting offer with the certainty of getting a title while playing for a big club like Real Madrid.