Spoilers Star Trek: Deep Space Nine

[Umbran]

But even then, Odo goes back to his family - initially to cure them, but also to join the Great Link and leave Kira (and everyone else) forever. Family trumps everything.

It isn't really about his family, though. It is about the entire species that is going to die if he doesn't return. And, if he's going to do that, he ought to stick around and teach the Great Link that not all "solids" are a-holes, and that the Founders were largely the architects of their own problems with solids.