Is there ever a good reason...

[BestestBrownies]

...for a 41yo man to have not had a relationship lasting more than 6 months?

That's it in a nutshell.

I recently met such a man. He seems lovely in every other respect, so is this a red flag or am I just being overly cynical?

I suppose it's possible that he's just never met the right person, but like you I'd be giving it the big .

Depends on so many factors.

How those relationships started and ended.

Whether he just wants brief attachments because he gets bored easily.

Whether he wants monogamy.

Whether they generally dump him.