IDK, I don't necessarily think it's a bad sign. Are you worried that they aren't being genuine? Otherwise, I don't think people can always articulate love.
For me, I have loved people--and seriously known that I loved them from the bottom of my icy heart--but I didn't know why. I couldn't explain, still can't explain it, but I know that they're in my heart. And sometimes it's people I don't even like 
(not that you should be with someone who doesn't LIKE you, but I'm just saying that I think love is more complex than always being articulable or being about what we want to feel).
If he actually made the statement "I love you but I don't know why," then *sideeye* because that's weird to phrase it in that way and wtf? Games. But if he says he loves you and he can't respond to a question of why when you ask, I don't think that's a problem---now there may be other problems that you know are there--but I just think that's an honest answer. If he was trying to be dishonest and smooth then he'd come up with movie lines like Boris Kodjoe in that Tyler Perry movie where he was like "I love how your hair falls on your cheek, or how you tilt your head to the side like so, or how you're a strong black woman of God." Um...no. lol.
I may be going on here about a situation that it totally doesn't apply to, but I think the nature of love (especially unconditional love) is that there isn't always a REASON or a RATIONALE. Like if someone asks why you love your mom (not that it's the same type of love that you have for a romantic interest, but still), you may give a reason or you may not, but I bet if the reason wasn't there (because she provided shelter for me, for ex.) you'd still love her because it's just there. It's just what you feel and it's unconditional for the most part.
Now I do think that long term relationships should be based on more than romantic love, so if you're asking if it's a problem that he can't articulate why he wants to be financially/legally attached to you in a marital sense--in terms of how the realities of life will play out, then yeah, that's questionable. But I wouldn't expect somebody to be able to list out a reasons of why they have romantic love for me.
Because at the end of the day, if for some reason the reasons weren't listed anymore, I would still expect that heaux to have love for me
ETA: Oh, also, so far in my life, I'm usually the more standoffish male-type (um...only in terms of feelings
) in the relationship, so take what I have to say with a grain of salt. I could see an ex complaining about the same thing about me, so maybe I'm wrong. Maybe me and your dude are wrong! lol
(please note I thought this a great answer, and it was much better than saying I dunno 
) It didn't make my feelings any less valid, because I couldn't articulate them better.(I am a numbers person, and I really not one for a lot of fancy wording) 