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In my D&D group, we have 4 great players, but one of them is constantly getting bored of their characters.
The other players are all invested in the story more than anything else and as such it really grates to have a character just drop out mid plot-arc and be replaced by some brand new guy who instantly needs to become best buds with the existing party.
However, on the occasions that I have refused him, or at least told him to wait a while before switching, he plays really half-heartedly, doesn't seem to enjoy himself and puts a damper on the whole table.
What can I do to fix this situation?