I would write,
1. "He has no friends for unknown reasons,"
2. "He has no friends for the obvious reasons."
In (1) the reasons are unknown and assuming that they weren't mentioned before in the text, they are undefined, which makes me not use the definite article. However, in (2) the reasons, although not mentioned before in the text, are obvious, which means they're obvious for both me and the listener/reader, so I'd put the definite article before "obvious reasons".
I'd appreciate your comments on the above way of thinking. Am I right to use the definite article in the second sentence and not in the first one?
I agree with you.